This is an obvious one. You ignore the little voice inside your head that says "it's too long, I think we're going to run out of yarn, didn't you start decreasing by this point on the other sleeve?" But you persist, pushing through the doubts. You tell yourself not to second-guess your choices.
It is at this point that you may catch yourself re-reading the pattern a few more times. That's a dead giveaway that you know something's wrong, but you can't quite admit it to yourself yet.
The most important thing about summer knitting projects is that they should be small. It's great to knit an afghan in winter, because the whole thing puddles on your lap and keeps you warm as you knit. It's like you get to use the afghan even before you have finished it!
But this is the last thing you want in summer.