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.
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.