On the development system, the program would only crash, under a heavy load, on the order of hours (like over 12 hours, sometimes over 24 hours). On the production system, on the order of minutes (usually less than a hour). But never immediately. The program itself was a single process, no threads what-so-ever. Core dumps were useless as they were inconsistent (the crash was never in the same place twice).
I do think that valgrind (had I known about it at the time) would have found it ... maybe. It might have caught the memory corruption, but not the actual root cause of the memory corruption. The root cause was a signal handler (so my "non-threaded code" was technically, "threaded code") calling non-async-safe functions, such as malloc() (not directly, but in code called by the signal handler). Tough lesson I haven't forgotten.