The D&D Cleric, apart from its Blackmoor origins as a vampire hunter (as noted here), is a mish-mash that has grown into a trope of its own.

The D&D cleric as a trope, encompasses:

• undead hunter
• healer
• second rate combattant
• priest of a pantheon (or faction within a pantheon)
• non-direct-damage spellcaster
• no edged weapons

The Cleric was one of the three Original D&D† classes; the other two were Fighting Man and Magic-User.

The Cleric as evolved to add priest of specific cults - the original presentation almost ignored the deity.

In fact, the original published presentation is short enough to quote:

Clerics: Clerics gain some of the advantages from both of the other two classes I Fighting-Men and Magic-Users) in that they have the use of magic armor and all non-edged magic weapons (no arrows!), plus they have numbers of their own spells. In addition, they are able to use more of the magical items than are the Fighting-Men. When Clerics reach the top level (Patriarch) they may opt to build their own stronghold, and when doing so receive help from "above". Thus, if they spend 100,000 Gold Pieces in castle construction, they may build a fortress of double that cost. Finally, "faithful" men will come to such a castle, being fanatically loyal, and they will serve at no cost. There will be from 10-60 heavy cavalry, 10-60 horsed crossbowmen ("Turcopole"-type), and 30-180 heavy foot.

Note that Clerics of 7th level and greater are either "Law" or "Chaos", and there is a sharp distinction between them. If a Patriarch receiving the above benefits changes sides, all the benefits will immediately be removed! Clerics with castles of their own will have control of a territory similar to the "Barony" of fighters, and they will receive "tithes" equal to 20 Gold Pieces/ Inhabitant/year

(D&D Original Edition, 6th printing, Vol 1, page 7.)

So, we have hybridization to some degree directly in evidence

• The D&D cleric is aligned not to a deity, but an alignment. This is counter to almost all historical models, save those of the dualistic religions (Zorastrianism, especially).
• We have the Hammer Films anti-vampire hunter (Holy Symbol, Holy water, repels undead that others can't).
• We have the Medieval "Fighting Priest" of the Romances (ahistoric, but historically loved) coupled with the priest-friar tending to a fighting company as chaplain (historic, but best beloved because of the legends of Robin Hood containing Friar Tuck, who combines both).
• We have also priest as landowner - a nod to medieval abbots and bishops.
• The use of no edged weapons isn't explicitly given an origin that I've seen, but appears to be a "thematic" element arising from the early Christian Conciliar prohibition on clergy causing the shedding of blood, coupled to a lack of grasp that blunt weapons also are quite capable of drawing and shedding blood. (A grazing mace will in fact rip you open nicely if you aren't armored.)

More evidence of mashup are the titles for the levels:

Acolyte
Adept
Village Priest
Vicar
Curate
Bishop
Lama
Patriarch

(D&D Original Edition, 6th printing, Vol 1, page 16.)

Acolyte, Vicar, Curate, Bishop: All used in western churches, including Roman Catholic, Church of England, and Lutheran (tho' not all Lutheran Synods).

Patriarch is used in the Roman Church for 4 particular Archbishops; its use is far more common in the Eastern Churches - The Eastern Rites of the Catholic Church and the Orthodox Churches. Orthodoxy also uses Acolyte, Priest, and Bishop.

Adept and Lama are borrowed from elsewhere -

• Adept from a variety of pagan traditions,

• Lama from the Tibetan... for Abbot.

  Which is itself an interesting placement - Abbots rank below bishops in Orthodoxy and Catholicism - but in the Early Irish church, were the ones appointing bishops.

Notably missing are the subdeacon, deacon, archdeacon, dean, and archbishop of the Catholic and Orthodox churches, and the High Priest common in pagan traditions, with the gaps filled by other traditions' titles. A sure sign of mish-mash. (Evil High Priests are listed as anti-Clerics in Men and Magic, p. 34, and on p. 9 under alignment tables under Chaos)

Later editions, in addition to expanding the spell repertoire, add priests of specific mythoi, and priests of specific deities within a given mythos. They also provide alternate titles.

D&D has had an influence, however. Fighting priest-spellcasters have become an accepted trope in fantasy fiction, even tho' the majority of priests in the Sword and Sorcery genere were one of three facets: Bookworm preacher, spellcasting priest, or warrior monk; all three can be found in the Conan stories. The D&D cleric has migrated into the video games and into a few Manga and Anime series. It has infected later RPG's, as well - the Shugenja of L5R has more in common with D&D clerics than with Shinto or Buddhist monks.

† Ignoring supplements for the moment.

Not all role-playing game have fumbles. In fact, I suspect that were an intrepid soul to catalog every RPG—a daunting if not impossible task—, more RPGs would lack fumbles than possess them. However, many games have optional rules for fumbles for those players who like them, and many games will have specific elements that'll see a deeply flawed attempt yield consequences worse than mere failure despite lacking a general rule for fumbles. So, yeah, while the games you've played have had fumbles, fumbles are by no means a universal.

Possibly the original fumble mechanics

While I'm no role-playing game scholar, Chaosium's Runequest (1978) apparently included fumbles at least as early as 1980 (which is the version I extracted from my shelf, dusted off, and cracked the binding of when I flipped through it). Runequest (1980) has the following section:

Fumbles

An Adventurer using a weapon for which he has only a 5-20% chance of success has a 5% (a roll of 96-00 on D100) of fumbling. For every additional 20% capability an Adventurer has with a weapon, his chance of fumbling with it is reduced by 1%. However, an attack roll of 00 is always a fumble. Even an Adventurer with a 100% of hitting (actually 95% for dice rolls of 96-00 are always a miss) will fumble if a 00 is rolled. (20)

(It took a moment to find the page number: they're on the outside upper corner of each page. Pro Tip: If you're laying out a book, don't do that!)

A chart on which one is to roll if a fumble occurs appears soon after.

Jonn Rees apparently ran these numbers both for skill use and the chart. I'm looking at Steve Jackson Games' Murphy's Rules (1988), a collection of comic strips detailing amusing and incongruous game rules from the magazines Fantasy Gamer and Space Gamer (yes, there used to be several gaming magazines!), and John Rees submitted an oddity in 1984 that says

In a 30-minute Runequest battle involving 6,000 armored, experienced warriors using Great Axes, more than 150 men will decapitate themselves and another 600 will chop off their own arms or legs.

So, even then, while the sheer frequency of fumbles wasn't considered necessarily bad design, it was, at least, considered funny.

Speculation: Why fumbles exist

Fans of role-playing games may view the fictional reality the role-playing game creates as a heightened reality, which I'll call for convenience reality-plus. In actual reality—the one most of us are in, like it or not—, very little that happens matters in the grand scheme of things. In reality-plus, however, everything that happens during the game matters, and spectacle is expected for good or ill. And spectacle during even everyday tasks can lead to comedy or tragedy that is a hallmark of reality-plus.

For example, while training with his Great Axe [sic], Argath of Sartar chops off his own head. That's a thing that probably would not happen in actual reality—very few real-world highly trained axemen accidentally decapitate themselves—, but that sort of spectacle is expected in reality-plus because Argath of Sartar is important, and his training took place during the game.

A game with fumbles tends not to mirror reality because the random number generator that's used has so little granularity. A highly trained archer might shoot himself in the head with an arrow or some other absurdity, but that surely won't happen once every 100 arrows he fires in actual reality! Were there even a 1% chance of every attempt leading to catastrophe in our reality, archery ranges would be sad, sad places, littered with the dead, naked, and injured.

But in reality-plus if those 100 arrows are fired while they matter—during the actual game—, one of those hundred arrows is likely to cause the archer's armor strap to break or make him fall and twist his ankle (those are average results on the Runequest (1980) Fumble Table). That happens in reality-plus because that's spectacular.

That possibility of spectacle makes the game, for some, more interesting rather than more ridiculous. It turns the game from what some may view as a mere simulation into a drama, albeit, in this fan's opinion, in a really forced way.