Did the EMdrive have a solid understanding of why it would work, rather than just being a discovered curious effect?
What's interesting about this one is that there is a theory at least. Experiments aside, someone whould be able to either confirm or reject the theory on its own.
The EMdrive also had a "why." Note, if you investigate these sorts of devices, they typically have a why. It's just that the why tends to ignore a bunch of other things and it's a disappointment in the end.
Really, if you're curious, you should dig into the history of all kinds of devices like these, anything in the Impossible Class. The inventor spends decades grinding away at it, getting inconsistent, odd little results which are just tantalizing enough to keep them stumbling forward.
I have often considered a taxonomy of these sorts of enterprises, with three orthogonal systems of reckoning. The first, a type really, is what principle is being violated (Second Law of Thermodynamics, Newton's Third ...). The second is psychological: misunderstanding, self-deception, or duplicity (typically to bilk inventors), with the understanding that the inventor typically starts with misunderstanding and proceeds onward. The third would, finally, be the discovered actual source of the odd results, which could be bad experimental design, not accounting for oddly-radiating services, vibration, friction, and so forth.
What's interesting about this one is that there is a theory at least. Experiments aside, someone whould be able to either confirm or reject the theory on its own.