I have learned the hard way, not all breakers are the same.
While doing some electrical to support my server rack, I opted to grab a few 30a quads from my lazy-person goto source, Amazon.ca.
I needed a 120v-30a and a 240v-30a circuit so the quad made for a great way to do this with one 120v 30a pole leftover for future use.
I could not get the breaker to fully seat on the busbar. I double-checked, yep, the panel supports QT type breakers, this quad is a QT breaker… /sigh
After contacting Siemens, I discovered that Canadian loadcenters are similar but not the same as US loadcenters, and thus a QT breaker in Canada is different from a QT breaker in the US.
So lesson learned and be warned – make sure to buy your breakers from authorized distributors only.