During QA for a huge project when our dev team was confident of the stability of the project, We started introducing small bugs, QA team use to raise bugs in Jira, we marked them as not reproducible.

Frustrated QA started coming to our cubes - at this point dev team worked in a perfect coordination like a man to man marking in hockey. While one dev asked QA guy to reproduce the bug in front of him while the other dev has already fixed it.

Continued for a couple of days till our team lead was satisfied with the revenge.