I think it's not necessarily complacency we're talking about, unless you're really hard on yourself and count letting yourself be distracted by time pressure.
For me, rushing and letting other people make me hurry have been the #1 reason for different "situations" in diving, by a large margin. Luckily, the worst (this far) was doing half a dive with the manifold closed, and the rest have been mostly forgetting to connect the primary light and such. Embarrassing nonetheless.
Lately, I've been trying to make a point (mostly to myself) of taking my time in preparing for the dive by mixing the gases, assembling kit, packing and checklisting (and re-packing) early enough. It's been nice; a lot less stuff gets left behind and I actually have time to do all the small fixes before having to fix them on-site.
Also, when doing the pre-dive-checks I've been trying to make it clearly my time - I'm a pretty laid-back guy (off-line, at least), but I'll at least try to snap a bit at someone if they interrupt my head-to-toes on the boat and start all over again.
Maybe it would help if we made students say aloud "man, would it be stupid to die because of skipping this" before doing any critical pre-dives.
Boat diving is the place where things tend to happen, in my experience, as it's always a bit crowded and there's a some time-pressure too, to get out of the way of others, at least. It would be interesting to hear how people manage this in places with really tight timing, we don't really have to care about slacks/tides in the Baltic.
Generally, at least in the club I dive in, new divers are given enough time to prepare slowly and carefully, but once divers themselves feel they should have a "routine" (at around 100 dives) they'll start to hurry and rush their buddies. When training dive leaders we try to get across to them that it's then their responsibility to slow things down, but quite often it's the new dive leaders who are doing the rushing, as they want to get diving started.
//LN