Pretty insane in my opinion. I can't imagine anything I would grade more harshly than not having a control except ethics violations.

I think there might be reasonable theoretical grounds for it in this case, though? If I was testing say a medical treatment or self-help technique, then yes, there should absolutely be a control group since some people might get better on their own or just do better for a while because the self-help technique gave them extra confidence.

But suppose I give people a pre-test, have them play for some minimum time, and then fill out the post-test when they're done. I don't see much in the way for random chance to confound things here: either they know the things needed for solving the tasks, or they don't. If they didn't know enough to solve the problems on the first try, they're not going to suddenly acquire that knowledge in between.

Besides, don't most university psychology experiments with volunteers keep the protocol secret throughout the whole experiment and then debrief at the end?

To some extent, but usually they still give some brief description of it beforehand, to attract people.

Alternatively, have you thought about doing a crossover-style design?

That's a good idea, thanks.