Anachronism

Anachronisms were objects, ideas or persons present in a time which was not related to its own, and thus liable to create a time paradox.

Regulation
The Time Lords' First Law of Time set out to prevent anachronisms such as a person meeting other versions of themselves (TV: The Three Doctors) or returning Eldrad to her present, by that time (from a 20th century point of view) the distant past. (TV: The Hand of Fear)

Serious instances
When anti-time began to bleed into the universe, anachronisms began to affect history and had a disastrous effect on the Web of Time. (AUDIO: Neverland)

Examples of their creation
The Monk had, by the time he met the First Doctor in 1066, caused many deliberate anachronisms in Earth history to "improve" it. (TV: The Time Meddler) The Fourth Doctor caused anachronisms of his own by deliberately defacing copies of the Mona Lisa, writing with felt tip pen on the canvas, "This is a fake". The words were readable in the 20th century after X-rays had been discovered. (TV: City of Death)

While in 18th century France, the Tenth Doctor thought he may have "invented the banana daiquiri a few centuries early". (TV: The Girl in the Fireplace)

The Rani created anachronisms in the alternate Rome on Terra Nova. (PROSE: State of Change)

Upon seeing a Sontaran teleport pod in the Rattigan Academy, the Tenth Doctor told Luke Rattigan that "it might be Earth technology, but that's like finding a mobile phone in the Middle Ages", pointing out that the presence of such futuristic technology was an anachronism. (TV: The Sontaran Stratagem)

Examples of their prevention
The First Doctor warned Barbara Wright against attempting to change 15th century Aztec culture. (TV: The Aztecs)

Since portable stereos did not exist in the 1960s, the Seventh Doctor called the ghetto blaster that Ace carried around with her in Shoreditch in November 1963 a "dangerous anachronism." (TV: Remembrance of the Daleks)