Less accidental; more like undocumented. BIOSes have used Unreal Mode, EMMs/XMSs can or temporarily use Unreal mode, Intel's SMM use it. There's even some evidence modern processors still support & use it.
One downside to Unreal Mode is that leaves the processor in in a state where naughty or non-standard TSRs, Interrupts, BIOS ROM code, and programs can cause hard lock ups.
One downside to Unreal Mode is that leaves the processor in in a state where naughty or non-standard TSRs, Interrupts, BIOS ROM code, and programs can cause hard lock ups.