It happens sometimes, your skills are better fitted to get the job done than the person assigned to do the job. What I try to do however is persuade them to let me teach the skill to the person assigned the job instead of taking over the job temporarily. In many cases, it doesn't work lol.
Oh yeah for sure. I always use it as an enablement opportunity for the person I'm working with! At the very least I document it all as well as I can and give them info so they don't have to pull me into it if they have it happen again. It works some of the time but better than nothing!