I had my doubts about this, but it's actually true. From Wikipedia:
The first cans were robust containers, which weighed more than the food they contained and required ingenuity to open, using whatever tools available. The instruction on those cans read "Cut round the top near the outer edge with a chisel and hammer." The gap of decades between the invention of the can and can opener may be attributed to the functionality of existing tools versus the cost and effort of a new tool.