Learn English – What’s the phrase that is commonly used after someone says or does something embarrassing/funny that you won’t let them forget

Consider the phrase rush to judgment as the action of one who concludes too quickly on the basis of insufficient or misconstrued evidence.

In an article published at stubbornthings.org Jerry Richardson contrasts a rush to judgment with the concept of the Rule of Law expounded by Madison in Federalist No. 51:

Vigilantes often rush to judgment, and get it wrong. Result? Innocent people get killed or otherwise unjustly punished.

I prefer this expression as generally accessible to less cliched than "judging a book by its cover."

The closest proverb I know of is Give a dog a bad name (and hang him). It is usually seen from the dog's point of view: once you have a bad reputation, you will be blamed for everything that goes wrong. But equally of course, if 'you' are looking for someone to blame, it is easiest to find somebody you already disapprove of.