This isn't a consistent problem, but it is an annoying one.
I ask people to register and log-in before commenting. Twice in the last week or so two different commenters have tried to log-in to leave a comment, but the log-in screen takes them back to the blog without them being logged in. One tried to log-in through my meta list on the sidebar and got to the dashboard, but then when she clicked to visit the site she wasn't logged in any more. She was able to do some back door things, but I can't expect my mom to figure out back door log-in tricks.
Again, it isn't consistent. The first subscriber hasn't had that problem again and has commented since. And my mom hasn't had the problem before today but as commented in the last week or so.