That reminds me how I was called to fix some Java service, which was successfully in production for 10 years with hardly any incident, but it suddenly started crashing hard, all the time. It was of course a thread safety issue (concurrent non-synchronized access to hashmap) which laid dormant for 10 years only to wreak havoc later.

Nothing obvious changed (it was still running a decade old JRE), perhaps it was a kernel security patch, perhaps a RAM was replaced or even just the runtime data increased/changed in some way which woke up this monster.