A derail is a caption or drawing that a troll or a user has drawn or written to deliberately set the game on another course, rather than simply interpreting the given drawing or caption. Often, Derailers do not comment in an attempt to not get in trouble, however, it's quite evident when someone derails a game. Derails are against the rules and is one of the reasons why there is a skip button as some players who don't understand the drawing/caption or who simply don't like the topic have a tendency to derail a game, although this is not always the case. It is also why there's a limit of plays you can do at a time.
Unintentional derails Edit
Self-inserts or inserting off-topic characters into whatever scenario it's portrayed by a drawing or caption can sometimes lead to a derail. For example, if someone writes the caption of "a man with a dog", and the artist draws Superman with a dog, this can lead to what some users would regard as a derail. However, many arguments have broken out over people mislabeling people as derailers. If someone chooses to create a game with the very first caption being very vague, then someone misinterpreting it should be expected.
What isn't a derail Edit
There is a big difference between of not getting a reference and a derail. If someone draws Olaf from Frozen and a user who has never seen Frozen writes "Snowman", that's not a derail, but if someone draws a picture of the sun and someone writes "candy cane", then it is. As mentioned earlier, misinterpretation isn't a derail.