I've seen this too and for me it always happens when i'm playing battlegrounds. Your theory on people joining while i'm in combat makes sense as i'm nearly always in combat in BGs and people AFK out/join late/switch BG etc.
I've had another problem as well with the party frames: Occaisionally they just won't update at all when someone joins the party. For example, someone will invite me so we're the only two people in the party, but then the next three people he invites will never show up in the frames. Last time it happened i tried a /reloadui and the frames came back, but the default bliz party frames also appeared underneath them. It only fixed itself after a relog. It hasn't happened enough for me to pinpoint the cause, but i could well have been in combat when joining the party like the error with the ghost frames.
Originally Posted by Gourd
Over the past few months (I'm unsure as to what version of pitbull I was using when this started happening), I have been having issues with the class colors on my unit frames. Occasionally when I switch target, the health bar (which I have colored by class) will remain the same color as that of the class I was previously targeting for ~5 seconds, although the name of my target will be displayed correctly as will their portrait. I have been trying to determine whether or not there certain circumstances that make this more likely to occur, but as of yet I have not been able to notice any trends or cause this to happen on demand. It occurs very inconsistently, but is highly frustrating - has anyone else who has experienced a similar problem been able to determine how to solve this?
I'm having the same problems here. They are very frustrating because valid and fast UF are very important for me (and of course in general)
This picture summarizes the problems quite well: (it's me I am targeting)
Any Idea how this problem could be solved? Does PB4 has the same Problem?