1. "Undefined behavior" due to typical memory bugs: code written in 100% safe Rust cannot trigger this behavior.
2. "Undefined behavior" due to a stack overflow: this can happen in safe Rust, but you can provably guard against it by banning recursion and dynamic dispatch, and doing static stack depth analysis on your program.
3. Panic/abort: naively written bare-metal Rust typically has many potential calls to panic!() due to bounds checks, integer overflows, .unwrap(), etc, and if these get called the system will typically fail catastrophically. I like to solve this by having CI fail if the optimized binary contains ANY call-sites to the panic handler. This forces the developer to handle these edge cases with idiomatic error handling before they can merge their code (but can cause some development pain/brittleness when the optimizer heuristics change).
4. Infinite loops: in most bare metal systems, if a section of code doesn't complete within some reasonable time, a watchdog will fire and the system will crash. Ideally there would be some static analysis that could tell me whether some particular code is capable of exceeding the allotted time bounds, but I'm not aware of such tooling, and need to resort to hacky solutions such as fuzzing and handling resets "gracefully" at runtime.