I once submitted a bug fix for an obscure issue to valgrind. They asked for a test case, which I managed to provide, but I was a bit nervous as I couldn't immediately see how to fit in their test suite.

The response from Julian Seward was so nice it set a permanently high bar for me when random people I don't know report bugs on my projects!

We still run our entire testsuite under valgrind in CI. Amazing tool!