Major incidents aside, I always think that cache-related bugs are some of the most likely to go undetected since if you don't test for them end-to-end, they're really not that easy to spot & diagnose.

An article sticking around too long on the home page. Semi-stale data creeping into your pipeline. Someone's security token being accepted post-revocation. All really hard to spot unless (1) you're explicitly looking, or (2) manure hits the fan.