I am not aware of such limitation but recently while preforming some custom upgrades to our farm I experienced some strange things (or not?).
Our development team is sharing Site Collection Admin account. When two of us are connected everything works as expected (browsing, simultaneously activating/deactivating features, editing pages etc.). Then third member joins in (just browsing with same account) and hell breaks loose. Randomly one of us cannot access anything on site and only error available is 'Page not found' (not even SharePoint access denied). Since there is nothing logged and since we don't receive any SharePoint error pages I suspect this is more IIS or NLB thing (we are running 2xFES with NLB). Or is it something else?
We were able to reproduce this behavior.
Does anyone have any good advice on this? I will accept even 'Don't share accounts' since we already stopped with this practice.
Farm info: MOSS 2010, 2 x Front-end servers in NLB, 1 x SQL 2008 r2, 1 x Search, Claims based authentication (Windows + FBA)