If you maintain widely-used software, you're going to get a lot of bug reports which are in fact user error, and you don't have a great deal of time to spend on each report.
So it's easy to mistakenly dismiss a report when it contains at least one user error, or even something close to a user error, even when it also exposes a real bug.
(It's less forgivable to continue being loudly wrong once other people have pointed out real problems more clearly.)