Now before people go saying "you can turn that off" hear me out.
Just recently when I get a private message it pops up like normal, I read it then reply or close. Then I continue browsing the forums, if I hit back or click on the 3D Spotlight OpenBoards link that private message pops up again, the only way I can get it to quit is to delete the message, but I usually keep them around for a bit, or I have to hit mark all forums read, I don't want that cause I haven't read them all.
Last week I'd get a private message and read it then I could go back to the boards page and I wouldn't get the popup again. Was something changed? Or did I just get lucky to not have this happen to me.
I guess I'll actually turn the popup off if this is a permanate thing, but I did like getting a popup when I actually recieved one. Getting a popup after I've read it though is kind of annoying.

Has never happened to me before but sounds like a 'cookies' problem, anyone experienced this too?

Never happened to me....

What browser(s) are you using?

I haven't experienced this problem as of yet.
One pop up per message and one only...
I always opt to open them in a new window and then close the window afterwards...

IMPORTANT: that you either check you PM's @ the beginning of each session, or enable pop-up PM window. Mods or other members may be trying to PM you!

IE 6 in XP

It happened for 2 days and now all the sudden it has stopped happening, so I guess its not a problem anymore.

I could write some comments about IE6 but decided not to..
Anyway, sometimes I forget to allow JavaScript to open new windows. I disable it so that those popups won't appear (in various sites).
Mozilla 0.9.7

That's funny. Lucky it did not happen to me.