I am running the SeeMeCNC customized version of Repetier on a Rostock Max V2. I have experienced this problem with both the stock configuration along with an E3D The Chimera. Sometimes, when I am printing and connect a mains device, my printer will fail in some way. The first time this happened I was printing with ABS, so I connected my fume sucker (A ~25-50W fan) to an outlet. However, when I connected it, the LCD showed garbled characters and my software (MatterControl) reported an error. Restarting the printer returned it to the previous state. I had a ~15 foot (coiled) extension cord connected to a 3-way splitter.
Today, I connected a hot glue gun, and MatterControl reported that Repetier had switched to "Printer set into dry run mode until restart!" (no heat or extrusions). Looking at the firmware, this seems to be caused by a thermistor failure. I had everything plugged into the same setup as before, except for a surge protector at the end. The circuit that it is connected to has a 20A circuit breaker, with probably ~3A of it constantly used.
My theory for this is that an additional device causes a slight fluctuation in the thermistor wires, causing an error to occur. However, the Rostock Max is designed so that wires are inside of aluminum tubes, which I think would provide EMF protection. The power supply is a generic 12V 30A power supply that was included in the kit, usually seen in LED strips. Both of these occurred while printing. I have ordered a UPS for the printer: would its under/over-voltage detection help? Are there any other solutions for this?