From Team Fortress Wiki
Disguising is an ability in the Team Fortress classic series of games that is used by the Spy. Disguising is used in order to sneak past enemy Sentry Guns and unaware enemy players. Upon selecting a disguise, "Disguising..." will be shown on the lower left side of the HUD and the Spy will begin to go undercover; this process takes approximately five seconds. Attacking with any weapon will instantly drop the player's disguise, but throwing a Grenade will not remove the disguise. Using grenades while disguised is a wise tactic when a team is having difficulty removing a Sentry Gun. Often, an Engineer will guard his Sentry and kill any Spy that threatens it, but it can still be destroyed if a Spy manages to set off 2-3 grenades. Team support is essential to this tactic to consolidate gained ground and get rid of the Engineer.
A player can disguise as any class, except the Civilian, on any team. A Spy will typically want to disguise as a member of an enemy team to trick the enemy team and enemy Sentry Guns. However, disguising as a friendly teammate has its advantages as well. It can be used to fool the enemy into thinking a more serious threat is coming, i.e. a Spy disguising as a Heavy Weapons Guy or a Soldier. As they leave, the Spy can then surreptitiously penetrate the enemy base disguised as an enemy. Also, disguising as a friendly player can be used in conjunction with Feign Death to sneakily get behind and backstab enemies that pass your corpse.
- The disguise actually evolved from a glitch in Team Fortress where players of the same team would sometimes appear as an enemy player. The source of this bug was found and adapted into an ability for a new class, the Spy.
- When disguised as a player of a certain team, all teammates and enemies will see the Spy disguised as that class on that team (contrary to what happens in Team Fortress 2). This also applies to the Spy's own arms in the viewmodel.
- When disguised, the faux health and armor values are always consistent.
- Disguising as a class with a different footspeed will not increase nor decrease the Spy's running speed.
- The Spy's disguise will always be holding the default class weapon (ie. Grenade Launcher for Demoman or Rocket Launcher for Soldier, etc).
- Spies will still bleed and grunt when taking damage while undercover, although in most Team Fortress Classic servers friendly fire is on. Effects are not transferrable to teammates, however, every so often a Medic or Pyro will try to infect or ignite their "teammates".
- If a disguised Spy dies, the corpse will be that of the disguise until that player respawns. Upon respawn, the model will change to that of the Spy. If the Spy selects a disguise, enemies can watch the player's corpse as it will gain the same disguise as the live player selects.
- Feign Death does not cause the Spy to lose his disguise.
- Being touched by an enemy Scout or Spy will instantly break disguise.