over heating

What you had happen is called a boil-over. Your engine may not have actually overheated, you may just have a problem in the coolant system. In a boil-over, the reservoir cap is where the pressure is relieved - the expanding and hot coolant needs somewhere to go. Basically, the coolant system did its job in keeping your engine or your radiator from going BOOM from the extra pressure.

There are several things that could cause a boil-over. The simplest is a coolant system that isn't sealed properly. Yukisho's suggestion is dead on for that.

Physics 101: coolant under pressure holds more heat per degree of temperature than coolant not under pressure. Essentially, your engine will overheat faster if you leave the radiator cap off - the coolant will pour out of the cap long before the engine is in any danger.

With the system under pressure, the coolant can take more heat away from the engine and dump it out through the radiator. If your hose clamps aren't tight enough they may leak or simply not let the system maintain the appropriate pressure to operate right.

As Yukisho said, check all your hoses for cracks and any signs that they are leaking. Also, tighten your hose clamps. They should be tight enough that you can see the hose clamp squeeze the hose. the best way to test this is with a coolant pressure tester, you may be able to rent one from your local auto parts store. The Mustang coolant system takes 16psi.

Your radiator cap may also be bad - it's intended to hold 16psi. Any higher, and it opens to let coolant expand into the overflow. If it lets off too soon, the coolant will overflow out of the overflow tank.

Report back after you've tried these things, and we'll troubleshoot some more.