Lenny's "fixed smile" surgery disappears after the first act.
Waylon Smithers is allergic to bees, but gets stung a few times apparently without consequences.
The black ball Mr. Burns throws disappears after he shouts at the other old people to leave his park.
There's no way Homer's broker would have allowed him to sell his stock at such a low price when it was obvious its value was going to continue to rise. He would stand to make a commission on the final amount, so he would surely have talked Homer into waiting.