Exception emask 0x50 sets all registers except for stack frame to 0x50. This emask should be set when the first instruction is a conditional jump. Usually, that instruction will be the first instruction, therefore, it is not needed. But if you have an instruction that is not a conditional jump, and if the execution of that instruction is dependent on the stack pointer, the emasking of 0x50 is needed.
Exception emask 0x50 is used to emulate the effect of a compiler error (such as a syntax error in a function call) which should be set to 0x50.