I often have a mental picture of the thing I need, I start writing it, get a bit "stuck" on architecture and think I could be using a ready made library for this. I find one or a few of them, look at the code (which is obviously more generic) and realize it's many times as large as I thought the entire project should be. With few exceptions the train of thought doesn't even reach the "Do I want to carry this around and baby sit it?" stage. Some how this continues to surprise me every time.
These 5 lines are probably my favorite example.