When does caching actually do more harm than good?

Lately, things have been acting a bit weird, and I'm starting to wonder if our caching approach is causing problems. We use Redis to cache API responses, but it seems like we're sometimes getting old data. Plus, getting rid of that old data is turning into a real headache. Have you guys run into common issues or situations where caching actually makes things more complicated and slower than they were before? What have you found works for cache invalidation, especially when you've got a lot of traffic?