This extension cord is rated for 15 amps, just like the connector, so technically it should be impossible to overload it if the circuit breaker is 15 amps. But the circuit is 20 amp!

So two heaters plugged into this, one on high one on low, will easily melt it without ever tripping any breaker.

But FWIW even within their rating they tend to melt, even with just 12 amps. I don't understand how they pass legal testing.