[fisherjeff]

I suppose this is one of those things where people’s experiences vary widely, but FWIW after 20 years of writing C/C++, I can count the number of times this has bitten me on one finger. And even then, I still think it was more likely just an optimizer bug in the god-awful compiler.

[yeputons]

Yes, I think it heavily depends on what you're doing and your mindset.

E.g. if you work with a single compiler on a single platform all the time and expect the compiler to "just compile the code", then you're likely to learn all its quirks, avoid them automatically, use proper abstractions/contracts/invariants to guard against incorrect code, etc. Moreover, you likely know how exactly the most popular UB shows itself with your compiler.

If you juggle compilers all the time and work with not-so-high-quality code (legacy or not), especially the one with little tests, I expect UB to pop up a lot. Or even if you just test on one platform and do final runs on another, that happens all the time in competitive programming or home assignment grading, especially with beginners.