Follow the process Tinga stated to make sure you mix your carbing sugar properly.
Overcarbonation happens for 3 reasons:
You added too much bottling sugar
You bottled too early
Infection
Or you are simply openning the beer too early, and/or not chilling the beer enough before openning it, which are 99% these types of threads.
Watch poindexter's video from my bottling blog.
Like he shows several times, even @ 1 week, all the hissing, all the foaming can and does happen, but until it's dissolved back into the beer, your don't really have carbonation, with tiny bubbles coming out of solution happening actually inside the glass, not JUST what's happening on the surface.
The
3 weeks at 70 degrees, that we recommend is the
minimum time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer.
But until then the beer can even appear to be overcarbed, when really nothing is wrong.
A lot of new brewers who tend to kill their two cases off in a few days, don't experience true carbonation and the pleasures thereof, until they actually get a pipeline going, and have their first 5 or 6 week old full carbed and conditioned wonderfully little puppy! Then the come back with an "aha" moment.
Everything you need to know about carbing and conditioning, can be found here
Of Patience and Bottle Conditioning. With emphasis on the word,
"patience."
I really shy away from the idea of infections or that even anything is wrong, if you've opened you beer earlier than the 4-6 weeks window...including gushing.
Stick the beers aside for another week or more, then make sure a couple of them are THOROUGHLY chilled for at least 48 hours to draw the co2 into solution. Then more than likely everything will be hunky dory....