The heatbed circuit uses two wires in parallel to carry +12V to the heatbed, and two wires in parallel to carry (switched) return current to ground through the MOSFET.
However, the connection of these wires at the heatbed end is subject to flexing fatigue. Wires that have solder in them are extremely easy to break. So what may have happened is that one of your two red wires or one of your two black wires broke off at the point where they are soldered to the heat bed.
One of the rules of electricity is that "current going = current returning."
Let's say that your heatbed draws 10 amps. Let's say one of the black wires broke. Now you have two red wires, each red wire carrying 5 amps but one black wire carrying 10 amps. That double current on the black wire will cause the black wire itself to get uncomfortably warm, and it will definitely exceed the rated current for passing through that connector at the board. Thus the connector will overheat.
So check the wires at the point of connection to the heatbed and fix if necessary. Then be sure you use a zip tie to immobilize the wires as they leave the solder joint. Let them flex farther out, but keep the solder joint stable.
Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick