An errorbanned user has errors inserted at random into pages they visit. You might consider this a more severe extension of slowbanning – instead of pages loading slowly, they might not load at all, return cryptic HTTP errors, return the wrong page altogether, fail to load key dependencies like JavaScript and images and CSS, and so forth. I'm sure your devious little brains can imagine dozens of ways things could go "wrong" for an errorbanned user. This one is a bit more esoteric, but it isn't theoretical; an existing implementation exists in the form of the Drupal Misery module.