In the embedded industry, the most common reason for things working when disabling optimizations is violating the ANSI aliasing rules. Embedded programmers type-pun all the time and don't realize it's undefined behavior in C. Most compilers have an option to disable the ANSI aliasing rules, but it really hurts optimization opportunities.
I saw a large codebase completely break when turning on intermodule inlining. The cause: more opportunities for the compiler to find pointers that aren't allowed to alias.
Yes, and most often it's clearly warned about, up to the point where lines are completely burried in type-casts, just to shut up the (well meaning) compiler. Because, you know, we thrive for 0-warning compilations!
I saw a large codebase completely break when turning on intermodule inlining. The cause: more opportunities for the compiler to find pointers that aren't allowed to alias.