what is the weirdest thing (library, framework, code snippet) in your opinion which you've used in production?

we are using a library that on every exception prints for us URL to StackOverflow related problem in the logs

Shame shame shame....

The stack overflow search is in my opinion a reason to not work with you guys.

To your question:

Weird...

Compression libraries.

I understand the basic principles but I'm always stunned and delighted like in my first christmas memories of childhood when seeing the compression results of eg. ZSTD.

Might be due to my lack of understanding for mathematics xD

Probably those home/trainee made frameworks that I somehow had to fix and send to prod.. These are unmaintainable and the worst nightmare the next dev will see.
No longer my problem tho