An overfill is caused when a player attempts to add a block to a column which is full and has none of its blocks forming a match. Doing so will cause the column to empty of all blocks, a substantial loss of points and the loss of all stored blocks and powerups in the case of Pointman and Vegas. In Pro and Elite modes you will also have to wait for your ship to respawn before you are able to collect blocks again.
During respawn time, it is still possible for Vegas to use its shuffle ability.
There is some leniency where it is not possible to cause an overfill for about a second after you fill a column up to the top - blocks moved into this column in this time are simply not added.
If you have Ironmode turned on and cause an overfill, you fail the song entirely and score nothing, being forced to end or restart the song.
Note that if you attempt to put blocks into a full column which has a match occurring in it, you will not cause an overfill, instead you will cause all matches on the grid to end and you will score points in the normal way for these matches. This can lead to a consecutive match, and is an immediate form of gravity match chaining.