Can anyone think of more in-jokes such as the thing with Lenny's eye:

1) Can of "Snakes" which Homer gives to him, one gets stuck in his eye.
2) The patch he wears and takes of to see "Santy Claus" (aka Father Fishmas). "Why Santy, why!"
3) The prank monkey episode where Homer throws pudding at him. "My eye! I'm not supposed to get pudding in it!"
4) The picture cube Homer gives him (as his secret santa) with the corners filed down so they don't hurt his eye. "It only stings mildly".

Any more/others?

Not so much a character in-joke, but I always enjoy the references to paramedics stealing patient's property.

Me Burns: With my last ounce of strength, I sucked out my gold fillings and swallowed them. Those paramedics have sticky fingers.

Lenny: You put that sticker on your car so you won't get any tickets. And this other one keeps paramedics from stealing your wallet while they're working on you.

There was something about not being supposed to get jigsaw in his eye too.