Fallen Parts

Fallen parts are any type of tank that had been lost during addition to the game. So far, 4 confirmed parts have been lost during addition, the first with the Switch.

Switch: Dreadnought
The Switch was a Dreadnought-class part that was to be added with the Pumpkin and Vortex. After modelling, the Switch was revealed to have been lost during transfer. It's stats are currently unknown and the only known thing we know of the Switch's existence is this image currently in the developer's decals. It would have been likely a Squadron-type when it came to projectiles, as it has 18 barrels facing three directions. The developer has stated it would have been a "layer-layer" style firing. The windows reveal it may have been four rooms and it resembled an office.

It could have been the first ever tank part that had hidden tracks.

Black Hawk: Predator
The Black Hawk has been mentioned several times. It's currently unknown what it looks like.

Superstriker: Battleship
The Superstriker was formerly a fallen part when it was added in 2.1 with the Clockwork, Chainsaw and Durr. It resembled a Star Wars Sandcrawler, and appears to have the "Hidden Barrel" style of attack.

it was finally readded in 5.3 in the form of a remodel.

Olympus: Carrier
Olympus is currently unknown to the public, although it is likely the star liner may have been it's appearance.

Trivia

 * So far, 17 parts have nearly entered the Fallen zone, and nearly 8 of those had been on thin ice.