Are bubbling preserved lemons a sign of spoilage? I recently made preserved lemons using a recipe from Jerusalem: The Cookbook. Having poked around a variety of recipes online, it's pretty much the same as all the rest: Lots of salt, squeeze out the juice, fill up the container with more fresh juice, add some rosemary and a red pepper. Float a little oil on top. Let sit in a cool, dark place for 30 days.

I opened the lemons last night, and a considerable amount of gas escaped from the lemon juice, which bubbled for about 2-3 minutes before calming down. The cap on the jar wasn't distended, and the mixture wasn't malodorous in any way. My instinct is to say, "This is bad; throw this away", but I haven't canned anything before so I'm not sure what to expect. What little information I can find on google is either contradictory or tongue-in-cheek and suggest it's fermentation.

What's up with my lemons? Are the gas bubbles escaping from the mixture a sure sign of spoilage?

EDIT: After a week, I'm still not not sure which answer to accept, as this, too, contains contradictory information... I suppose time and votes will tell.
This is totally not a problem.

This style of lemon preservation relies on fermentation. The salt is not intended to halt all fermentation- it just restricts it to the tasty kind. Fermented pickles are a common and traditional way to preserve food because the salt and acid and thriving tasty bacteria make a very inhospitable environment for bad bugs.

The production of gas is just a symptom of the fermentation and not a problem. I regularly make Indian lemon pickle, which sounds like a similar recipe, and it produces some amount of gas. Recipes will typically call for the lid to be gently closed to allow some of the gas to escape- just to prevent the bottle from exploding.

This is not your typical canning. Typical canning uses acid or high temps to stop all bacterial growth. In that kind of canning gas production would be a very bad sign and you would have to throw it out.