Much religious art from The Renaissance on. This had a solid Real World justification, though. Religious paintings, especially on the walls of churches were designed for the masses, and the goal was not to depict a scene exactly as it was, but to tell the story for everyone to understand. Through the use of contemporary clothing, armor, and styles, even the common people could instantly recognize "that's a soldier, that's a fisherman, that's a shepherd, that's a tax collector, that's a nobleman, that's a commoner" etc. instead of "WTF are those people in those silly clothes?"
This got completely out of hand by the 18th and 19th centuries, when both Pontius Pilate and Herod were depicted in extravagant Persian-type robes.
Historians can and do judge when forks reached different parts of Europe by looking for them in paintings of the Last Supper. Judging military equipment is a little trickier, as you can never quite predict when someone's depicting the cutting edge and when he's depicting a suitably "old-fashioned" type of armor, but that tends to be well-attested elsewhere.