I helped defend Whiterun (Imperial) and after few minutes I have received a Letter of Inheritance and 90 gold because Severio Pelagia died. I don't remember this guy, so I searched for him at the Elder Scrolls Wikia and found out that because of that I will have problems with some quests.
Are there a list of such quests that you "must-do-before-something" or it will become unable to complete?
Best Answer
You can see a list of best quest timing in The Unofficial Elder Scroll Page about it (UESP). This site not only suggest you the best sequence of quests to do but also tell you the pros and cons of doing that (including links to quest walk through and item details).