As to software bug - the offending piece may be not in the "offending area". Say, R1 contains a semi-random value from some previous process. Now you issue MOV @R1,#0 instead or MOV R1,#0 and your program may work fine (i.e. that was a non-critical delay counter) unless the value pointed at by R1 was important - and since the bit addressed memory is overlaid with standard scratchpad, a common byte write can change the bit flags just as well.

In C it's even more notable when you write to an uninitialized pointer or beyond the end of a string.