This morning while firing up my Linea for the first time after a rebuild and PID upgrade, the boilers were filling up when all of a sudden... BAM!!

My first thought was that something electrical had exploded and immediately I killed the power and turned off the water. Upon closer inspection I saw that one of the group caps had blown off the group from the built-up pressure. In my haste to test it out for the first time, I didn't bolt the caps back down to the portafilter locking rings. I'm not sure how far up in the air the cap flew, but I'm damned lucky that it didn't land on my head. As it was, the edge got dinged up a bit on the concrete floor of the workshop.

BOLT DOWN THOSE GROUP CAPS!

i humbly suggest checking the setting/operation of your OP valve before turning on again. good luck. After 7 years of using commercial lever's on a daily basis I recently hit myself in the face by not paying attention to a locking mechanism.