Star Wars The Rise of Skywalker. The movie was already getting a bad rep pre-release, and in response to potentially sales-damaging claims that Palpatine was coming back, Disney had Ian McDiarmid explicitly claim he wasn't. A bad movie where there was nobody to point a finger to became a bad movie where there was someone to do it to. Then he passed away shortly after. I witnessed this mess all go down in theaters.
Ian McDiarmid didn't pass away. But yes, this movie might be the single worst cinematic disappointment I've ever witnessed in my life. I'll never watch it again.