Error detected

Usually it is because you are heating the bed near it's physical limit and it took too long to get there. On my particular R2 my physical limit for bed temp is about 85° C. It can be set higher, but it takes so long to get there that I would get the same error. But since I don't print anything needing that high a bed temperature I don't really care that the bed heater can't be heated higher.

The second usual cause is that if you have a version 1 bed that has pogo-pins to electrical attachment one or more of the pins has failed, if you have a version 2 bed which has a socket on the bed, maybe the cable is loose.

 

Another factor which can cause the error is that the PID values are not tight enough. This situation can cause the heater (Bed or Extruder) to take too long to get to the designated temperature. Calibrating the PID values can fix this.