Consider following scenario:
- UserA adds a new class to a factory
- UserA does cache rebuild to make this class available in it's code
- UserA does commit
- UserB does update
- UserB does Deploy, which triggers same cache reset
- UserB sees a Fatal Error, that class isn't registered within a factory
- UserB does "Reset All Keys"
- all starts to work back again
Probably with Memcache enabled Deploy doesn't reset something related to new class additions and that causes a fatal error.