My worst code review involves line endings. In one of our products we use the facade pattern, and one of the facades is about 5000 loc.

Well, a dude in our team messed up with line endings, so our diff tool showed that 5000 lines have changed. A newb I was at that time, I didn't know any other diff tool, so I went through all the lines manually only to see that 2 lines effectively have changed.. and the newb I was, didn't thought about rejecting the change immediately! We then had somehow to revert that commit, but messed up our central repo (was still with CVS). In the end we had to restore the daily backup for the repo because we couldn't manage it to do it right.

Never ever had such a disastrous code review again.