[rules-users] Events in wrong order - will it cause problems?

I used to let drools set the timestamp of events automatically, but a new requirement forces me to use @timestamp for my events. Normally my events should come in the right order, but sometimes (as the events origin from mobile devices) it can occur that the order is incorrect. I tested with "wrong" timestamps and it seems like they would work alright - but I think I read somewhere, that events inserted in a wrong order might be ignored in some circumstances (especially by CEP rules). How can events in wrong order cause problems?