Why has my Smartlevel allowed my aquarium to be overfilled?

Overfilling is usually because of two situations:
1. The pump you are using to top up the Sump or Aquarium is too powerful: Hydor recommend a Hydor Pico 250lph (P16300) & 400lph (P16400) for sump refill and Pico 600/115lph (P14200) for Aquarium refill. Although the Hydor pumps seem small they are ideal for top up.
2. The sensor has been moved or is floating in the sump. This will cause overfilling, however if the magnet positioning is used correctly then the sensor should not move and therefore not create this situation.