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The airborne aircraft carrier "Valiant" in Doctor Who (around 2005, I think) was apparently designed by the Master when he was taking power on earth.
But when the Doctor beat him, time was rewound and that part of history erased.
Why did the Valiant exist anyway and why was it apparently well-known to everyone but the technology used in it not used anywhere else on earth?
Consider that one of the first lines after time unwinds itself is something along the lines of "We've just seen the President killed".
Time rewound to a point immediately before the activation of the Paradox Machine. However, the Valiant existed before that (after all, the Paradox Machine was aboard when it was activated).
One would thus expect the Valiant to continue to exist, since it did beforehand. As for why everything on board is as it was at the end of the Year That Never Was, the Doctor explains that the it was at the "eye of the storm" - the point on which the paradox was centred - and didn't revert for that reason.
Time is very flexible (one could say wobbly). We would see that the master created the airplane, but cause and effect isn't really so much important here.
Time will attempt to alter as little as possible to accommodate the new conditions. This is to prevent the butterfly effect. Another example of this is
when Amy's parents cease to exist, Amy does not
In both cases reality takes a blind eye to the impact of someone no longer existing and goes about its day changing as little as possible to keep the ball rolling.
An element of time-travel, parallel universe, and temporal-anomaly stories, the object and persons at the center of a paradox-event tends to survive and remember the event. As I was reminded by Darael the Paradox Machine was turned on after the Valiant was created, so even if the timeline was undone, it would only revert to a time after the Valiant was built.
Though the Master claims to have built the Valiant entirely, it is likely part of its design came from UNIT. The Master used a technology called a Paradox Machine, a part of a stolen TARDIS, to allow paradoxes to exist. When the machine is destroyed by Jack Harkness, the paradox destabilized and reverted to the previous unaltered universe.
In this case, the Valiant was at the center of the storm and even though it was part of a timeline that no longer existed, created at facilities which may no longer exist, its presence persisted in the new restructured timeline.
In the following year in an erased timeline, the Valiant was the base of operations for the Master. The Jones family (with the exceptions of Martha, who had escaped, and Leo) and Captain Jack Harkness were imprisoned on the ship, as was the Tenth Doctor. It also held the paradox machine inside the Doctor's TARDIS. When the timeline was erased, the Valiant was the only point in space and time not to revert, instead moving directly through time, due to it being the "eye of the storm", that is, of the paradox machine. (TV: Last of the Time Lords)
