I loved when my IDE would warn me that my code wasn't deterministic unless I used c++11 or newer compilers because previous versions technically didn't define how it should work, so every compiler handled it differently.

And all the times I had to specify C++11 because it had features I needed, and suddenly it was a huge headache because the testing pipeline wasn't REALLY compatible, it just said it was, and then handed it off to manual review. Something I didn't know until 6 months after I started using it...