the biner that broke here was on the non-tightening side of the line. a was set up to always be on the non-tightening side because there is pretty much a permanent girth-hitch on the carabine. nothing slowed it down when it failed at this point. there is so much tension in some slackline systems that i think the even if only a piece of webbing hit you, it could do some damage to you. . . let alone a piece of flying carabiner.
I had an anchor sling fail on me a few years ago. The carabiner that flew thirty feet into my knee only grazed me, but it left one hell of a bruise
. I doubt I'd be walking today if it had hit me dead on.
To the topic at hand: the stretch-relax cycles on a slackline will occasionally open a carabiner (I've watched this a few too many times to doubt it) so I don't walk on my line until every carabiner is a locker screwed shut. My tightening system is different, but it is backed up by the tightening system you can see in majid's post, which is rigged entirely with lockers in my system (and with an overhand on a bight instead of the clove hitch that m_s has).
Judging by the photo, this looks like an open gate failure, so replace the crabs in that location with lockers (I've found autolockers won't unscrew with vibration of the line, so that may be worth considering).