Setting up the filter paper properly is very important to successful filtering. If a crease is not added to the round filter paper, it will not fit as well as it should in the filter. This can lead to a loss of some of the fluid.
Even if the filter paper is set up well, overfilling the cone can lead to two errors. First, some of the fluid may be lost over the edge of the filter. Second, the fluid could easily go in between the filter paper and the filter, allowing the solid particles to get into the collected fluid.