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, continuously generate CO2 as one of their waste products. If an alcoholic beverage is bottled while the yeast is still active, the gas will build up in the bottle.
This is a positive effect when you want your beer or wine to be "sparkling", but if the beer/wine is bottled too early, the pressure can build up too much and cause the bottle to explode.
This is a Bad Thing(tm) when you have glass bottles as the bottles can explode and be a real mess to clean up, not to mention a potential hazard 9glass is hard to get out of carpet, especially when you have sticky beer making it hard to vacuum out).
The other unfortunate side effect 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 wine that you don't intend to be sparkling, make sure you wait long enough that the ferment is definitely over - don't just rely on a hydrometer, use some common sense. At least 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 stil lsome yeas thtere... be patient, the longer you wait, the better the wine 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 - mainly because it does effect the way a wine ages (makes it slower).
Don't just guess 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 9and 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 dn't know how much sugar to put in, you can generalyl get those "brewing sugar tablets" from the supermarket that give a pre-measured amount of sugar for each bottle... don't forget to read the instructions carefully - make sure you know what size bottle you're using!
It's a good idea to store pressurised bottles in a way that they won't set each other off (in chain reaction). A trick recently explained to me is to put them in a box together, but to then tip a few buckets-full of sand over the top - so that 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.
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 safty measure and cheap and easy to transport - though you may have to hide them at events due to the un-period nature of the container... of course you could just pour it into a decanter when you're ready to drink it...