Wikipedia:The final straw

From time to time, a user may engage in conduct which the community finds to be problematic. Warnings may be issued and there may be long discussion.

Occasionally, this conduct may result in a block, a ban or a topic ban. There will normally be a catalytic event: an edit after a final warning, say.

The catalytic event may not in itself be terrible. It may be relatively innocuous taken on its own. It will rarely be so egregious that it would, on its own, result in a sanction.

But it is the last straw.

The sanctioned editor may argue that the single edit was not in itself terrible. It may in fact have been entirely valid as an edit, but be a breach of other restrictions. These arguments routinely fail. The last edit is not the reason for the sanction, it is merely the catalyst.