Barbed nails aren't barbed like a fish hook is...they just have tiny teeth in them near the head to help prevent them from coming out of whatever they're hammered into. At least that's how any barbed nail I've seen has been (I'm no carpenter). From what I understand spiral twist nails are much more effective as far as not coming out is concerned.

A nail can be used in different ways without being barbed however. The nail could be the barb. If hammered through a wooden spear (or lashed to one backwards), it could serve as a barb when harpooning fish. Short nails can also be tied inverted for fishing so that when a fish swallows them they turn sideways and get lodged. Before fish hooks existed this is how men fished using natural resources like bones, or thorns.

Benjammin: I like your way of thinking. I hate to carry 'uni-taskers' if I don't have to...especially when most multitaskers are WAY more bang for the buck (like 10p nails)