That's what the Blinovitch Limitation is actually about. In order to create a paradox, you'd have to overcome the potential energy behind it. This would take a colossal amount of energy in the same way that leaving the planet's atmosphere would. Once that paradox is created (see The Sound of Drums), you're basically sitting on a temporal bomb.Lt.Snuffles said:
The Limitation says, in a similar way to Einstein's Theory of Relativity, that as you get closer to creating a Paradox, the energy needed to overcome it builds exponentially.
The TARDIS can overcome that though, by slipping out of the time-stream, but it's still incredibly dangerous, which is why the safety circuits are there.
The real problem with a paradox though exists depending on whether you're in a linear stream or a cubic (or more dimension) stream.
For a linear stream, Effect follows Cause, so the creation of a paradox would destroy the paradoxer (and probably the paradoxee).
For a cubic stream, Effect probably follows Cause, so you're allowed to create a paradox, but the ripples forward will disrupt your own time stream.
For the pie-eating, this may just be indigestion. But if you released Nelson Mandela, say, then the Anti-Apartheid movement would lose potential, this may mean that racial tensions increase to the point where Apartheid never is denounced. This then knocks on to the appearance of Black people in the media. Maybe Louis/Rochelle aren't created in Left4Dead because in this new world, Valve think that a black protagonist is too risky? Maybe Barack Obama takes a even worse battering and the American public bring in McCain? The World Cup is in jeopardy because it can't be held in South Africa, so it's moved to England early. The English Government can't cope with this after the credit crunch (fueled by ongoing racism against the extra immigration from South Africa) and martial law has to be declared.
It's all about potentials, and that's why there's certain key points in history where the Doctor fears to tread.
