I am using Forms Based Authentication and security seems to be setup properly. The problem I am getting is that every so often, my users report access denied issues on one of my site collections.
I'll log in as the user and can replicate the issue, then log back in as an administrator and simply run a check permissions for that user on any library in the site collection. The check confirms that the user has access, and from that point, the user will no longer get the access denied message. I'm not sure what happens, but seemingly a week or 2 later I'll get the same problem, easily corrected with the same check permissions routine.
Right now, I only have a handful of users right now but in the very near future I will be dealing with hundreds and soon on to thousand+, and cannot live with this manual fix as often as it is coming up. Any explanation/help is appreciated.