This story contains spoilers for Marvel’s Ant-Man and the Wasp and Avengers: Infinity War.
Ant-Man and The Wasp continues the longstanding Marvel Cinematic Universe tradition of post-credits scenes – the film has two in total, one midway through the credits and one at the very end – but unlike most of the previous Marvel movies, where the stingers were playful bonus scenes or Easter eggs for comic book fans, you’ll be missing out on a major plot twist if you decide to leave the theater as soon the movie ends… mostly because Scott, Hope and Cassie’s miniaturized drive-in movie night isn’t really the end of the movie.
The first post-credits scene finds Scott venturing back into the Quantum Realm to gather more healing particles to continue Ghost’s treatments, with Hope, Hank, and Janet staying behind to operate the Quantum Gateway tech, which is now stashed in the back of Luis’ van. The mission goes off without a hitch until Scott radios back to the regular-sized world for Hope and her parents to bring him back, and we see that all three have been killed by Thanos’ snap, leaving nothing but ash floating on the breeze as an increasingly panicked Scott tries to reach them. Ouch.
Source: IGN.com Why You Need to Stay for Ant-Man and The Wasp's Post-Credits Scene