Original author here. I think i wrote the sentence in the post confusing. The error rates of (40% 500 error) were not normal. A normal hit rate was at ~97%. But because the redis server itself was overloaded by KEYS request, BGSAVE / forking, etc. this instance was not able to answer the cache request. Due to this we had a high error rate.

But in general i agree. A normal behaviour of a cache with such a bad rate should be avoided. I hope this make more sense now?