Apparently I have done it wrong my whole life.
"[...] The ordinary method of cutting out a wedge is very faulty in this respect (N.B: avoid letting exposed surface become dry). The results to be aimed at are so to cut the cake that the remaining portions shall fit together." - Galton, 1906
Reference: http://www.nature.com/nature/journal/v75/n1938/abs/075173c0.html