Steps to recreate:
[LIST=1]
[*]Quest->Misc Data->Item Drop Sets
[*]Select any of the 256 item drop sets
[*]Click 'Edit'
[*]Type a negative number in "Nothing Chance:" text box
[*]Observe negative percentage appears in adjacent percentage label
[*]Click 'OK'
[*]Click 'Edit' again
[*]Observe negative number has been replaced with 16-bit two's complement
[/LIST]
The chance text boxes on the two tabs clamp the negative chances to zero after clicking OK, but still update the percentage label with negative percentages in the meantime.