Glass grenades

A glass grenade is the colloquial term for over-pressurised bottles encountered during brewing- more specifically, if said bottle goes pop with considerably messy consequences...

Yeasts, used in brewing, produce CO₂ as a waste product. If an alcoholic beverage is bottled while the yeast is still active, the gas will build up in the bottle.

This can be desirable if you want your beer or wine to be "sparkling", but if the beer/wine is bottled too early, the pressure can cause the bottle to explode.

This is a Bad Thing™, especially if the bottle is glass. Not only are exploding bottles a hazard, but they can be a real mess to clean up. Glass is hard to get out of carpet, especially when you have sticky beer making it difficult to vacuum.

What complicate matters is that if your bottles are near each other, one explosion can set off all the rest.

So, some basic safety measures are in order:

• If you are making a non-sparkling wine, make sure you wait long enough that the ferment is definitely over. Don't just rely on a hydrometer, use some common sense. Make certain that the wine is completely clear - clear enough to read through, for a white wine. Shine a torch through it and see if there is a scattering effect - if so there's still some yeast there. Be patient: the longer you wait, the better it will taste. It's better to wait now and have better wine than to lose the whole batch to explosions.
• If you are bottling sparkling wine, make sure you have pressure-proof bottles (eg champagne bottles) with the proper corks for bottling under pressure. You can also go for crown-seals, but you'll have to have enough personality to stand up under the pressure of people looking down their noses. There's still considerable prejudice about this option, due to the way it slows the aging of a wine.
• Don't estimate a time to bottle based on if you think there might be enough sugar left in the brew to get it to pressurise. This is a sure-fire way for the yeast to prove you wrong and keep at it longer than you expect. I would recommend that instead of guessing how depleted the sugar is and picking a time to bottle before the yeast finishes, you let the yeast finish - completely. Then add some more sugar when bottling to pressurise. This gives you two things: firstly, the safety of knowing and controlling how much goes into each bottle (and thus how pressurised it'll be), secondly, the consistency of pressurisation which gives you a gauge to work against for the next brew batch.
• If you don't know how much sugar to put in, you can generally get those "brewing sugar tablets" from the supermarket that give a pre-measured amount of sugar for each bottle. Read the instructions carefully - make sure you know what size bottle you're using! Another alternative is to use a sugar dispenser that releases a set amount of sugar.
• It's a good idea to store pressurised bottles in a way that they won't set each other off (in chain reaction). One method to prevent your entire batch from decorating the ceiling is to put the bottles in a box, then tip a few buckets-full of sand over the top. Each bottle is separated and cushioned from each other bottle. This sand will absorb most of the "blast" if there is a grenade, and will give you a chance to save the remaining bottles.
• If you don't need effervescence, and have particular problems with a batch, you can use a wine stabiliser such as potassium sorbate. Potassium sorbate doesn't kill yeast, but does prevent it from reproducing. It won't stop a fermentation, but does prevent yeast from waking up in the bottle.
• As a final alternative, if you are only going to be storing the beer for a short period of time, and again don't mind a few sneers... store it in the big plastic coke bottles. Not only are these rated for ridiculously high pressure levels, but if one goes pop, you have a split bit of plastic, but no glass shards in the carpet. I wouldn't recommend this for anything more than a few weeks as the plastic-taste is eventually going to be a problem (so not the solution for wines) but it is a great safety measure and cheap and easy to transport. You may have to hide them at events due to the non-period nature of the vessel (a cloth bag the size of your bottle works nicely). Preferably, pour it into a decanter or jug when you're ready to drink it.