Could someone describe the glitch so that I can avoid activatin it on future playthroughs?
Judging from what Arthmoor says, the script fires at the beginning of the game, so there is no escape. As of yet, we don't know what it mucks up exactly aside from taking of loads of script processing time, but it can't do the companions any good.
Changelog:
http://electronictheatre.co.uk/xbox360/xbox360-news/20753/elder-scrolls-v-skyrim-update-1-6-live-xbox-360
I'm 99% sure this is legit...
Probably taken from this bethblog post.
http://www.bethblog.com/2012/05/24/mounted-combat-arrives-in-latest-skyrim-update/