Invalid friction, sometimes shortened to IF, is a glitch produced by giving a block of solid ground a bad value for friction (i.e. a negative number or a block of text). Its side effects allow the creation of new types of maps with features such as teleporting, phasing through solid ground, and delayed springboards, among other things. These are all more of features than bugs, but are technically the latter.
Any non-ground object that touches IF ground gets teleported to coordinates (0,0) and is stuck there until the end of the round. This includes mice. One application of this is putting the mouse hole slightly below (35 point below) (0,0). Then, if a mouse gets cheese then touches the IF ground, it has successfully completed the level. By this rule, potentially any solid ground could act as the mouse hole. Here is a proof-of-concept map:
Once an object touches IF ground, objects that remain on any solid ground will be pulled downwards, eventually with enough force that they pass through the ground. An application of this is to divide the level into (at least) two parts. The mice start up in the higher section, then, when IF activates, they phase through the ground to the lower section. Interestingly, objects (including mice) that are not in contact with any solid ground are not effected by IF activation. It follows that the lower level must be made of objects like boards, boxes, etc. instead of solid ground. Proof-of-concept map:
When mice phase through solid ground when IF is active, they do so with great force. So, by placing a board underneath the solid ground (which must be thin), a springboard is ostensibly created. Proof-of-concept map:
IF is also used to disable red anchors in a map.
Make the IF ground small (10x10), make it dynamic, set its linear damping to 9999, select fixed rotation.
Place it in a location where it could not be quickly and easily reached and put red anchor in its centre.
Create another ground above it that is dynamic. Adjust distance and linear damping of it so they touch (disabling red anchors) on desired moment. Keep in mind that making the distance too small may result in connection before the red anchor is spawned and nothing will happen. Proof-of-concept map: