Try not to intrude.
If you come up on a combat situation in progress, try to refrain from just jumping in and saving the day. On occasion, people might have certain outcomes that have been established a head of time and it would be irritating and upsetting if you saved Jane from the mean and nasty Evilman when they had arranged for her to be kidnapped. There is nothing wrong with paging the combatant parties and asking if you can join in or even if it would be feasible for you to join in. This is where the lemming factor and the timestop enter in. If you choose not to use a timestops then feel free to OOCly notify any newcomers that the fight has been going on for a while and it would not be feasible for anyone else to join or that the final outcome has been prearranged and things need to work out a certain way. And please, the receiver of these OOC comments should not take them personal. Timing and planning counts for a lot and I'm sure no one would like to have their plans ruined even if it was all in good intentions.