It all depends, the type of gas may have something to do with it, but not really, I tried both regular and super and both did the same. I get no CEL's and it happens most when it is hot and you rev the car high. This means that your car is running rich and unburnt fuel is being sent into the Cat causing the foul smell as stated above. This can be caused by a lazy O2 especially if no CEL is visible. You can test O2's using a voltmeter I believe. The O2 should switch from .1-.9 volts or so (Do not quote me on it) quickly to operate properly. If it switched slowly, it is a lazy/dying O2 and if does not switch, it is dead (the CEL would warn you about the dead O2)
