The Daily Beast chronicled the tale of 50-year old metal head that went to see Motorhead and for two weeks afterward suffered a headache. What happened was the direct cause of what we all do at shows- head bang! Not that we all assumed head banging was good for your brain, but who knew it could cause internal bleeding and blood clots that will make your life basically a living hell? Head banging has been documented as such by doctors cited by the site.
“… headbanging, with its brisk forward and backward acceleration and deceleration forces, led to rupturing of bridging veins causing haemorrhage…” and declared that the case “serves as evidence in support of Motörhead’s reputation as one of the most hardcore rock’n’roll acts on earth, if nothing else because of their contagious speed drive and the hazardous potential for headbanging fans to suffer brain injury.”
The article goes on further to explain the situation and even talks about Slayer's Tom Araya needing surgery from head banging so much.
"Despite the verdict, Motörhead isn’t the only band that can inspire dangerous moves. And it’s not the first time the medical profession has asked metal heads to take care, saying it can cause injuries from whiplash to tears in the carotid artery—basically “shaken metalhead syndrome.” As the study’s author noted, there have been at least three other cases of brain bleeding from headbanging, including one acute case which led to the patient’s sudden death.
Even Slayer frontman Tom Araya announced in 2010 he was retiring his signature helicopter and figure-eight style thrashing. Doctors had implanted a steel plate into his spine to deal with the physical damage, 'from 30 years of being in the same position, headbanging,' Araya said."
We'd encourage you not to head bang, but then we'd be hypocrites.