As it has been noted many times, time travel is a subject best avoided in speculative fiction, due to its inherent problems with causality -- especially when you have to take into account events which you, as a writer, have not invented yet but which can retroactively become continuity busters. Even when the writers do bother to create a reasonable model of time travel, the temptation to divert from it for the sake of spectacle is just too high. Doctor Who is probably the worst offender here; not even trying to explain how time travel works, it goes for the "timey-wimey ball" explanation and chooses whatever fits the story for a particular episode.

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