What is Ruby method cache invalidation and why is it critical for performance?

Method cache stores resolved methods; invalidation occurs when class hierarchy changes.

Ruby caches method lookup results at call sites. If a method is added, removed, or overridden in a class/module, the cache becomes invalid. Ruby flushes or bumps cache version counters globally or per-class. This ensures correctness but can cause performance spikes in metaprogramming-heavy frameworks.