Sometimes purchased sheet cakes can be a problem to cut into uniform pieces so I always ask the bakery to put "hash marks" or even divisions on the cake top if it is not decorated extensively with an intricate flower pattern. The bakery pipes the small rows of evenly spaced icing on top of the cake and then inserts a very tiny flower in the center of each square.
However, I recently encountered a different problem with a decorated sheet cake from a bakery ~ the problem was that it was a two-layer rectangular cake with frosting flowers, a retirement sentiment, etc. Quite ornate. In order that the top not be marked up, I requested that the bakery put the "hash marks" or cutting lines on the four sides of the cake where they would not interfere with the top.
The next dilemma occurred as we tried to cut it with a standard cutting knife. A very long piece of dental floss was the answer. Two of us got on either side of the table, aligned the dental floss on the corresponding hash marks, and "cut" away. It worked like a charm with a minimum amount of mess and all the slices were uniform and very neat. And best of all, there was way less mess than if we had tried to continue using a knife.
By Ronsan from southwest MO