Pulling a nail from a spare piece of wood. Despite your vice-like grip, the pliers slip off, your elbow travels back at high speed and bruises into the bench vice. You drop the piece of wood which lands on your favourite scribing tool***** which just happened to be half on and half off the bench. The scriber is launched into a roly-poly base-over-apex flight across to the nether regions of the workshop where it takes 3 hours of moving the scrap about to recover it.
It would have been quicker all-told to have gone down to B&Q and purchased a new piece of wood to suit the job in hand, which time and inconvenience you were trying to avoid in the first place!
***** Even though you've got 3 other scribers that would do any job that comes along, it doesn't feel "right" unless you're using your favourite.