In the x86 architecture, there are multiple registers that are not widely used nowadays. These registers were used in DOS operating systems before virtual memory in the way we know it was introduced, particularly the segment registers. Apart from the FS register (which you already know about), there are other segment registers, such as CS, which was used to point to the code section, DS, which was used to point to the data section, and SS, which was used to point to the stack.
The pop SS instruction is quite special. This instruction is used to get a value from the stack and change the stack segment (or address) according to this value. So if there's any exception happening while executing this instruction, it could lead to confusion (which stack would be used to store the exception information?). Therefore, no exceptions or interrupts are allowed while executing this instruction, including any breakpoints or trap flags.
If you are tracing...