Universe-shaking developments that are pretty much never mentioned again.
Time travel is a particularly significant example, because genre series love one-off time travel episodes and don’t always tidy up afterwards.
“We’ll never make it!”
“Uh, Captain. We have a time portal. Should we maybe use that?”
I’ve used time-travelling MacGuffins now and then in not-about-time-travel series, and always take care to smash them at the end of the episode.