'flunked everyone who claimed they got the supposed "correct" answer to three significant digits because that was impossible.' while I've never seen anyone flunked for this, I certainly have taken off substantial amounts of points, and seen others do the same, for 3 significant figures when 2 is the absolute highest reasonably possible (and realistically, one sig fig was what we actually wanted).

I've run the exact lab you're describing, and I think we gave full credit for anything between 5m/s^2 and 20 m/s^2 provided there was some acknowledgement that this was at odds with what was expected. We very often would check in halfway through class and either tell the kids what they were doing wrong, or even tell them to write something 'this is at odds with literally all known science and I think I don't trust this'. For this particular lab, I've never seen errors as large as the ones you've described, so your lab was likely very poorly set up.

In other cases, I've made extra time (and allow students to come in) in case their numbers were so weird as to be problematic; just depends on the lab. Any teacher worth their salt will do this. It's a shame the teachers you had were terrible and incentivized bad stuff.

If being in a lab has taught me anything, it's that doing good science is often morally difficult. Sticking by your guns is hard.

But you are right in some sense: there are definitely incentives to... misreport. The best we can do as teachers is to reduce those as much as possible and reward kids/students for being honest.