[RPG] Benign Transposition on an unwilling creature

Resolve the edge case with an impartial ruling: random location via die roll.

Since the rules don't explicitly define what happens in a case like this, and since the class ability moves the PC into a place that is based on PC knowledge, letting go of the gridbound mind set is a simple way to resolve this unexpected encounter. This link is to a similarly inexact situation regarding players being spit out of a gelatinous cube.

or do they just "bounce off" into the next available space?

That's a fair and simple way to resolve this: the PC bounces of the invisible obstacle. The PC still gets to escape the current problem, but due to the ever present problem of dealing with "the unknown" something unexpected happens when taking this action.

• This is a common pattern in play at the table: for example when dealing with unseen traps or invisible barriers. The PC moves with confidence from "here" to "there" and something unexpected happens along the way

Die rolls are used to resolve uncertain results: use the available tool

The basic framework of "How to Play" is retained. (PHB p. 6)

1. DM describes the situation
2. Player describes the desired action
3. DM narrates results
   (Roll dice when the outcome is uncertain).

Roll a 1d8 (if using a square grid) or a 1d6 (if using a hex based map) and assign the number 1 to relative north (top of the map). Move clockwise around the grid or hex pattern to determine where the warlock unexpectedly ends up.

Example of a bump:

• On a roll of 4 on a d6, the warlock ends up directly below the hex she was escaping to. (relative "south" to the top of the map)

• On a roll of 3 on a d8, she ends up directly to the right of that space. (relative "east) to the top of the map)

  What does this ruling do for you(DM) and for the player?

• It keeps play moving

• It removes any hint of DM influence on the final destination

• It preserves the location of the invisible creature

• It provides a surprise/unexpected outcome due to incomplete player knowledge -- which is OK since the players are dealing with the unknown a lot during their adventures

  I've used this quick and easy tool for unexpected things like this (most commonly for our old "shooting into melee and you missed, what did you hit?" scenario in a couple of older editions) since around the time I began DM'ing.

In my experience, this way of making an ad hoc ruling based on an unexpected situation is both quick and fair. It has been with the game as long as it's been played. I learned it from DM's before me.

The dice are a tool. In this case, what the dice do is keep it fair and random. This also frees the DM from having to explain why the PC ended up on a given square. Fate/chance took over when the PC bumped into an unseen obstacle at its destination.

You're right, there's theoretically a slight disincentive to learn spells from the wizard's own school

You've hit on an interesting conundrum, which is that by making it cheaper to copy spells of a specific school, it's theoretically more cost-efficient to learn spells from schools other than the one you chose for your Arcane Tradition when you level up and then hope that you can learn the spells in your own school by copying them for half price. Essentially, every time you pick a spell in your own school instead of another school, you're shortchanging yourself, because copying that spell would have cost you less than another spell, so getting it for free has less value. If your only goal is to learn every available spell as cheaply as possible, you always want to choose spells from other schools on level-up.

However, this is assuming you can find scrolls or other copies of all the spells you want in order to copy them into your spellbook, which is far from guaranteed. Unless your DM is very generous with giving you scrolls as loot, the only way to guarantee that a spell will be in your spellbook is to pick it on level-up. So in practice, your best move is usually to pick spells based on how useful they will be to you in your adventuring rather than theoretical value. After all, if you cared more about gold than magic, you wouldn't have become a wizard, would you?

Lastly, I'll note that you don't necessarily need a lot of spells from your own school in order to get value from your spell school features. For example, an Abjuration wizard can always activate their Arcane Ward simply by casting Mage Armor at the start of their adventuring day, and doesn't need any other abjuration spells prepared. Obviously they would get slightly more value if they occasionally recharged their shield by casting other abjurations throughout the day, but they will still get value casting only one abjuration per day.