I hate when I start something and then think "you know, I bet someone's already done this and done it better than me", look and then find that its true.
I hate it more when it happens on the scale of hours or even minutes.
I have gotten to the point that if I find a problem that no one seems to be solving - I wonder if it is the wrong problem ... or at least improperly formulated.
While programming problems these days might not be so academic, or theoretical as they used to be, there are still plenty of things to solve. Why does this program do strange things, how can we glue these components together, how do I make this program do Y when it does X.