(Video contains NSFW language)

It wasn't a very happy birthday for Motley Crue guitarist Mick Mars, who got knocked over by an overzealous fan on stage during a concert in Canada on Saturday night (May 4).

The legendary metal group were performing at Spectra Place in Estevan, Saskatchewan, Canada on Saturday, which also happened to be Mars' 62nd birthday. They were right in the middle of performing 'Primal Scream' when an audience member jumped on stage and ran into Mars, sending him sprawling and making a grab for singer Vince Neil before security jumped in and hauled him away.

Mars suffers from a painful medical condition called ankylosing spondylitis (AS), which slowly fuses the bones together. He was diagnosed when he was 19, and the disease has gradually robbed him of much of his mobility, making an incident like the one Saturday night more critical for him.

"With most people it stops in their 30s, when the disease is still in their lower back. In my case, I have a rare form of the disease that hasn't stopped, so it's all the way up my whole back and into my brain stem," Mars told MusicRadar in 2008 (quote via Blabbermouth). He added, "It's literally squeezing my rib cage together, so I've lost some height. And now that it's up in my brain stem it's hard for me to move my head in any direction, be it up and down or sideways. So I can't drive, I can't do any of that kind of stuff — this isn't a 'boo-hoo', it's just the facts."

His band mates certainly didn't take kindly to the incident. As the offending audience member was dragged away, one of the musicians -- either drummer Tommy Lee or bassist Nikki Sixx, from the sound of it -- yelled into the mic, "What the f--- is wrong with you, you f---ing idiot?"

Neil added, "We might be back," as the band left the stage to get Mars checked out and make sure he was okay. They returned to the stage later to finish up their set without further incident.

The Crue kicked off their current Canadian run on April 21 in Penticton, British Columbia. They'll play a string of dates in the US on May 16, and in September the group will return to the Joint in the Hard Rock Hotel & Casino in Las Vegas for another dozen shows.