A Spycrab, a commonly known Team Fortress 2 fad, is a player Spy with their Disguise Kit out, crouching and looking up. It is considered by the community to be an "endangered species".
The most common of them is an "Oblivious Spycrab". They are usually unaware/aware but don't care that they have wondered into a battlefield. However, they usually distract the enemy because of its stupidity.
This version of a spycrab is usually known to keep to themselves, like most of them. However, when approached or attacked, they will disappear (using cloak) and reappear, only to backstab most of the enemies.
A DSc will cross the enemies line of fire and try to distract them. If successful, they will bait them into an environmental hazard or a small area, making them victims to backstabs without too much commotion.
A gathering of spycrabs. They will all usually act like a regular Spycrab, but when one is provoked, a large number of them will chase after you, though they're easy to escape/kill.