If you ender into the corner of a block you can clip into it and continue to jump up and out of blocks. This can get you into peoples bases or get you from cave system to cave system (with enough regen and gapples). It is really abusable on online servers. It can affect anyone and can be abused by people playing factions or UHC servers. It is bad if you accidently clip into an object and dont know how to get back out.