Character Sheet Modification

Interesting thought I had. During a course of a rp, something might happen to a character to hinder it, like they suffer some crtical injury that never heals right, or some ability that they gain that helps them, like they gain anti-magic ability. Would these things be added on to a character sheet as time goes by, or do the player themselves have to keep track of the changes their characters. 

And would this type of change be even allowed?

Changes would always be allowed, but use common sense. Though, I should clarify that character sheets are not to be edited unless you ask a mod or admin.

Thanks for the info!