I've had similar experience when the compiler immediately found unsynchronized state deep inside a 3rd party library I've been using. It was a 5-minute fix for what otherwise could have been mysterious unreproducible data corruption.
These days even mobile phones have multicore CPUs, so it's getting hard to find excuses for single-threaded programs.
Multithreading had not been planned or architected for, it took 30 min, included the compiler informing me I couldn't share a hashmap with those threads unsynchronised, and informing me on how to fix it.