While some devs probably find this an acceptable, even humourous attitude, end-users who are stuggling to explain a bug they've run into may feel infuriated if that's your response.
I know as well as you do that fixing a bug that can't be reproduced is near on impossible sometimes.

For those not familiar with testing in general, if you can't reproduce a bug, how can you:
(a) identify the bug ?
(b) fix it ?
and
(c) prove that you've fixed it?

Internally, its fine to joke around and say "it never happened" but all reports for this "phantom" bug need to appended onto the original bug report. If the bug truely exists, eventually, you'll have enough dots to draw a line between them and identify the bug...