Many passes in LLVM tend to assume that the IR is in some sort of poorly-defined canonical form. For example a loop transformation might bother to only handle loops where the latch and exiting block are one and the same, or assume the existence of a dedicated loop preheader. In principle, this dependency on canonical form should only be one where code is less optimal if the code isn't canonical, but compilers are big, complex things, so it should be no surprise that sometimes the noncanonical IR ends up causing crashes or miscompiles.
Do you remember a case where it did cause a crash or a miscompile, and it wasn't also a case where that same crash/miscompile could have happened under default/optimal phase order?
I can easily imagine that reordering phases increases overall likelihood of surfacing bugs that are there anyway.
I have a harder time imagining a dependence on canonical form that leads to a crash under phase reordering but is totally sound otherwise.
The cases where they're the most likely to occur is in the codegen pipeline, but codegen is already acknowledged to be a situation where you have non-optional passes that have to occur in a particular order, so I suspect you're already discounting those scenarios.
