Last week I was working on a web development assignment (php and bootstrap).
I had a few problems but the most annoying was that bootstrap didn't want to apply to one specific element.
I sat there, looking for the error for about 2 hours. When I finally found it I was raging inside. I mistyped a - instead of an = after the class attribute (i.e. class-"Foo").
The mistake was hard to spot because of a combination of the font and the low screen resolution - 2 hours wasted because of that shit