I had to deal with a concurrency bug in Ruby once and it was so bad* that it pushed me into Elixir, which makes the vast majority of concurrency bugs impossible at the language-design level, thus enabling more sanity.

Ingeniously simple solution for this particular bug though.

*as I recall, it had to do with merging a regular Hash in the ENV with a HashWithIndifferentAccess, which as it turns out was ill-conceived at the time and had undefined corner cases (example: what should happen when you merge a regular Hash containing either a string or symbol key (or both) into a HashWithIndifferentAccess containing the same key but internally only represented as a string? Which takes precedence was undefined at the time.)