Undefined instruction exception


We are using ARM Toolset , Toolchain MDK-ARM Professional  Version 5.21a and Device is LPC2388. 

We are trying to create a "undefined instruction exception" on a "BL" instruction in the thumb mode. When we tried to alter the H fllag in the first part of the instruction, we were unable to single step through the instruction as the instruction decoding was not successful. But when we moved the H flag altering to the second part of the instruction, the single stepping went through the instruction and proceeded to the next.

I wonder how on both the occasions the undefined instruction exception was created. This might cause a controller reliability issue in a mission critical system.

Can someone explain why the undefined instruction exception was not entered in our case and suggest a way of creating undef-exception on a BL instruction.

Thanks in advance.