Wikipedia:Al Capone effect

The Al Capone effect is the tendency of editors that cannot be held accountable for long-term disruptive conduct to eventually be blocked for a violating a less serious but bright-line rule. Behaviours like civil POV-pushing, well-meaning incompetence, and chronic low-level incivility can do serious cumulative damage to the project, but have proven difficult for the community to stop. On the other hand, there are comparatively minor offences that by convention result in an indefinite block, such as abusing multiple accounts or repeatedly engaging in edit-warring. Editors will often repeatedly evade sanctions for the more damaging misconduct only to be finally excluded from the project for one breach of a lesser rule.

This probably happens because, on the one hand, editors are more likely to break bright-line rules about edit warring or sockpuppetry when they are pushed – by repeated failed attempts to sanction them for their chronic behavioural problem, instance. And on the other hand, the probability of coming back from a block is inversely proportional to how much far you've exhausted the patience of the community. This is a dangerous combination for the would-be Al Capone because, while their latest victory in a lengthy ANI or ArbCom case might make them feel like they can get away with anything, what it actually means is that the community will treat that fourth revert on list of Dora the Explorer episodes as if it were the Saint Valentine's Day Massacre.