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.

Convincing Google that I don't want its assumption that I want a typo fixed is getting more difficult.

The dynamic Google results page means that it's really difficult to refine a search based on the presently visible results which disappear as I update the search. I find that that behavior incredibly annoying, and greatly appreciate that DDG doesn't do this.