Unfortunately this happens way more than anyone would like to admit. The amount of times I stuttered on an word while talking to a coworker and they mocking do it back to me right after is astonishing. I’ve honestly learned to just ignore it and continue on so that they feel like an ass hat when their “joke” isn’t even acknowledged. Edit: also I don’t know if you’re an Office fan, but “did I stutter” is a famous line by Stanley, so they may of been quoting him and that context.