Temporal paradox

Temporal Paradoxes were featured prominently in Father's Day. The paradox in Father's Day was when Rose Tyler saved her father, Pete Tyler. There was also the problem that an earlier Doctor and Rose Tyler were watching this change in the timeline.

When the paradox occurred, the earlier versions of the Doctor and Rose Tyler disappeared and the inside of TARDIS disappeared. There also seemed to be unusual occurrences such as a song being played before it was made or the first phone call being repeated. As well, the car which should have killed Pete Tyler, kept on following him in order to repeat the accident. Eventually, the reapers were able to break into normal reality, where they tried to eat all the people they could find.

Pete Tyler was able to fix the paradox by getting killed by the car. This caused all the damage that was done by the reapers and the paradox to repair itself. Though there were still changes to the timeline, Pete's death was apparently enough.

The Doctor mentioned that when the Time Lords were around, there were ways to stop the creation of paradoxes and that any paradoxes that did manifest could be fixed. This might mean that paradoxes weren't possible or common before the Time War or they had different effects.