Anachronism

Anachronisms were objects, ideas or persons present in a time which is 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 themselves (DW: The Three Doctors) or returning Eldrad to her present, by that time (from a 20th century point of view) the distant past (DW: The Hand of Fear)

Serious instances
When Anti-Time began to bleed into the universe anachronisms began to affect history and had adisastrous effect on the Web of Time. (BFA: Neverland)

Examples of their creation
The Monk had, by the time he met the Doctor in 1066, caused many deliberate anachronisms in Earth history in an effort to "improve" it. (DW: The Time Meddler) (The Doctor and the Monk had known each other while growing up on Gallifrey previously. PDA: Divided Loyalties) On the other hand, the Doctor caused anachronisms of his own by deliberately defacing copies of the Mona Lisa with "this is a fake" written with felt tip pen on the canvas. The words would be readable in the 20th century after the X-Ray had gotten invented. (DW: City of Death)

Anachronisms were present in the Alternate Rome created by the Rani on Terra Nova. (MA: State of Change)

The Daleks introduced anachronisms in many times they visited, including 1866 (DW: The Evil of the Daleks), 1930 (DW: Daleks in Manhattan), 1963 (DW: Remembrance of the Daleks), 1984 (DW: Resurrection of the Daleks) and other times visited by the Daleks after the Last Great Time War

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

Since portable stereos did not exist in the 1960s, The Doctor called Ace's stereo that she was carrying around in Shoreditch in 1963 a "dangerous anachronism". (DW: Remembrance of the Daleks)