Thomas came across this Thermal Master TM-350-PMSR ATX power supply:

Maximum output power is given as 350W but if we add up all of the individual output power ratings we get this:

 12V x 10A  = 120W
  5V x 14A  = 70W
3.3V x 14A  = 46.2W
 -5V x 0.5A = 2.5W
-12V x 0.5A = 6W
  5V x 2A   = 10W (standby)
____________________
Total       = 254.7W

Is it a mistake? Is it false advertising?

Turns out that there is a post on the forum at badcaps.net about these power supplies and it is only a 250W power supply.