Usually it's turning an uncommon domain specific word into something uselessly common (and completely unrelated) for me.

Or when I put an entire error message in quotes and it deems the number of results for that error message too low to be intentional so it deconstructs it into a useless mashy search of all the words in the error. Note, I don't mean when the results are zero, but even then I usually have to spend an annoying amount of time before I realize that my search actually had zero results instead of the millions it claims it had.

There was a time when google's cleverness was just enough to be useful, but it gets more and more clever (and frustrating) every year now.