Spikes - Spikes can appear on any block and will hurt both heads.
Lava fireballs - Fireballs that can jump up from the lava and can hurt both heads.
Coloured fireballs - Green and pink fireballs that travel in a straight line which only hurt the opposite head.
Lava - Any head that touches the lava results in game over. Some levels the lava is stationary, some levels the lava tide goes up and down, and some levels they continuously rise and force the player to be very quick.
Gates - Some later levels have gates which go up and down and will hurt any head. The player has to be very careful to pass through them while they're up, and not make contact with any metal part of the gate.
Coloured pegs - The player must use the correct colour head to connect with the peg, the wrong colour results in losing one health and unable to connect with it.
Coloured blocks - The player can step on the blocks but if they use the wrong coloured head they will lose one health.
Coloured fireballs - They travel from one tube to another and can hurt the player if they touch the opposite coloured head.
Knuckleheads contains unused regular text, unused text from Dirk Valentine, Go Go UFO, text that is present unused in other Nitrome games but never actually used in a game, and what is presumably placeholder text.
Knuckleheads contains an unused full colon, comma, and an unused question mark (?). Although a question mark is used in all levels, this type of question mark is different and looks more like the regular text found on the game's highscores submission page.
Of unused text pertaining to Go Go UFO, there is the text for a starting and ending bracket, and a ninety degrees flipped plus sign, making it look a bit like a cross. Of unused text pertaining to Dirk Valentine, there is a forward slash, quotation mark, short hyphen, and a British Pound symbol.
There is what seems to be a beta or placeholder image of a heart in a circle, this likely meant to indicate a heart capsule in early versions of Knuckleheads. It is the only non blank circle image that is an .svg.
Finally, Knuckleheads has two unused images of text, the first reading "where's my coin Jon?" and the second reading "where's my health Jon?". Jon likely refers to Jon Annal, the artist for the game.
Unused recurring backwards slash that is not used in any game