>that's the one time the GC decided to run. Good luck debugging or even reproducing that.

GC usually runs on allocations, so it's quite predictable. The amount of time spent on collection is pretty much as indeterminate as the time spent on malloc/free.

When will a correctly-implemented GC ever alter your program's functional correctness, aside from timing-related bugs that aren't fixed by manual memory management either?

Just run your program normally and use eBPF/bpftrace with something like a flamegraph to track your system and its GC. This isn't an easy problem by any stretch of the imagination, however, it is a solved problem.