Dealing with the many scenarios that could break replays was problematic, but being able to run several instances of the same game at the same time and compare

Dealing with the many scenarios that could break replays was problematic, but being able to run several instances of the same game at the same time and compare them occasionally for desyncs can help alleviate that. For example, using a pointer value (address) as a hash table or sort key means that depending on allocation order, iteration of game entities may be different from one run to another.