Yup. I watched a good friend of mine in a pickup game go down once. He just casually said "welp, that's my ACL". He gets up, walks off the court (no limp), packs up, leaves the gym. Goes to the doctor the next day...yep, torn ACL.
Everyone has this vision of Derrick Rose laying on the court and thinks that's what ACL tears look like.
I played for a while, then walked off when I tore mine. Wasn't that painful, just felt weird. Like I had no stability in my knee.