Mixing North American Panels and Breakers
The magic snap is not magic. Contact blades are not flat.

One thing you will notice about US breakers in matching panels is that they "Snap" in with a definite resounding snap.  And when you try to remove them, the initial force needed to pull them out the first 0.1mm is considerably greater than the force needed to pull them the rest of the way.  

That's because the bus bar in the service panel actually has a little "nub" shaped into it, of a specific shape.  That shape engages matching curves on the breaker's spring clip.  It works like a detent, creating the snap.  However the good contact also creates a lot of mating surface area.  As such, it runs at sane temperatures during high current flow, which is why UL approves it.  

Every panel manufacturer uses a different shape of bus bar, requiring different shaping on the breaker's spring clip.  They are different because of patents.  Each manufacturer has patented their shape, because they think it outperforms their competitor's shape (and avoids violating their competitor's patents). Eventually patents expire, such as Square D's QO (queue oh) patent expired, permitting Murray/Siemens to manufacture its QD (queue dee) breakers for QO panels.  Still, Siemens is unable to make breakers that fit both Murray and QO panels, because they are still different.  They are locked into the old designs because new breakers need to fit old and new panels alike.   

So when you use an alien breaker, the clips and nubs don't mate as intended by the two manufacturers, and this is not an accident, it's by design. They are not trying to keep you from using alien breakers, they are (were) trying to keep from being sued for patent infringement.  Anyway, you end up with the clips only mating at 2-4 points instead of flat surface contact, and point contact like that is only good for a couple of amps before it starts melting, opening, arcing and burning.