I've done stupid things before like going on the internet in order to find out something about a game that I thought was relatively innocuous (example: looking up the voice actor for a certain character) and then accidentally spoiling the entire game. Usually by going the character entries in Wikipedia (right underneath where said voice actor is listed), and reading the character descriptions and learning that so and so dies. Ridiculous. Definitely learned my lesson in that regard.
What really does grind my gears is going to forums to chat about a certain game/anime/show and reading comments that don't give ANY warning whatsoever, and drop a huge spoiler- especially if its a series similar to Game of Thrones, where there's a book/manga series which is ahead of the timeline as compared with the show....and someone starts to discuss events in books 4/5 when the series is only effectively on book 3. Definitely uncool.