Have you ever read a story where a Sue sings hip-hop in Middle-earth, or refers to "Istanbul" in the Colonial Era? Do Renaissance characters ever say "okay", "you suck", or "what's up"? These are all examples of anachronistic cultural contamination and form the backbone of PPC charge lists in many "olden days" continua.
Anachronistic cultural contamination is a step above your average historical inaccuracy. These most often consist of knowledge, devices, or geographic locations that should be nowhere near the inhabitants of a certain timeframe. It is usually a Mary Sue who introduces these into the continuum, and with them come a host of problems. If one sings a modern song in the 1700s, it's not the Apocalypse, but if one brings in advanced technology, such as pistols that can fire multiple shots without reloading... well. The very fabric of history has been contaminated, and agents must take care to correct all such errors in their missions.
One of the worst offenders in this area was caught by the Department of Technical Errors on Redwall's island of Sampetra. This medieval setting was contaminated with cell phones, e-mail, electronic gates, spy cameras, walkie-talkies, and bombs. Read about it here.
Please note that, should agents become careless, they may also be responsible for anachronistic cultural contamination, should they accidentally leave behind HQ technology or knowledge. To prevent this, use of the Anachronism Detector is recommended.