We have a 3.2.5 site which is for registered users only, and we occasionally run into a user who just cannot get logged in. Usually we can trace it to a browser problem (Netscape 6.1 has bad mojo and must be upgraded at once). But there is the occasional user where we just cannot figure it out.
I think it has always been IE where we have had to give up and say "use someone else's computer", but I put that down to these being mostly corporate users more than it being an IE specific problem. It happened once with someone using Lotus, too. *shudder*