Accidentally biting inside of lip - hard

[pdq]

CJsNvrUrly wrote:
I tell you once.
I tell you plain.
What you have done,
you'll do again.
You'll bite your cheek,
careful or not,
upon the already bitten spot.

One of the perverse facts of the universe; once you have bitten your lip, it will swell up and make it even more likely to be bitten again. Only far more painfully.