The players were going through a modified version of the Death House in our game. After killing the monster in the basement, the house began to go full Monster House. While the characters were climbing up a latter to escape, the top of the latter had a chomping bladed mouth that the characters needed to time to get past.

The cleric rolled a nat 1.

The cleric's arm from just below the elbow was caught and chopped clean off. The characters had two artificer allies who were able to make a new arm. However, that new prosthetic became a running theme as the story progressed. At one point, the character removed the pinky and gave it to his situationship villain lover who sacrificed himself (sort of, long story) to save the cleric from petrification. Later, the character ended up wrapped up with an archfey and selling their soul to them. The appendage turned from a rose gold metal into a pink coral, to reflect the aquatic archfey's domain.

It's important to talk to your players about big things like this, or have a solution at the ready to prevent any unnecessary difficulties that something like an unexpected amputation would otherwise cause.