This is what can happen when a careless builder runs a 20 amp circuit backstabbed through a 15 amp receptacle. Though some devices may be rated for a higher pass-though current, backstabbing does not provide more than two single points of contact and, thus, is not a good idea. In this case, a chest freezer was running downstream of this while a clothes washing machine was doing laundry using this device, and the breaker never popped. Good thing someone was home and awake when this occurred....
If you've got a 20 amp circuit and a 15 amp device, it's nice to run it through wire nuts instead of the device.
Or better yet, put in a 20 amp device.