A few notes on the matter... First off, NOT COOL, DUDE. I can't believe someone would jump on stage at a Britney Spears concert. #1- You're not at a Metallica concert or in some small club or bar and #2- You KNOW Britney is fragile and unpredictable. Why would you do that to her? How did this kid even get that close without someone pummeling him?
Look at the genuine fear on her face. It's kind of upsetting. One, because she is so spooked and two because she is seemingly unable to handle the unplanned nature of it. Not that you can really compare Britney Spears and Solange Knowles, but the younger Knowles sister calmly handled a drunken fan who decided to join in one of her recent performances (watch the clip around 3:20). Britney bugs out then just goes half-heartedly back into the routine.
Lastly, now that we know Brit Brit is safe, can we talk about how she appears to let out a blood-curdling scream into her face-mounted microphone and we hear nothing?! I know that it's widely accepted that Britney lip syncs her entire show (heck, I was there and heard it in person) but it's kind of a slap in the face to know that her mic isn't on at all during the performance.
The whole thing goes to show that we're still dealing with a very fragile individual.