How much fermentation happens post-bottling in a UFO? In Harpoon's UFO series (and other related products from other companies), the beer is bottled unfiltered, so some of the yeast carries over into the bottled beer.

As long as the beer is kept in a cool area, as not to promote the growth of the yeast, how much fermentation will occur?  Aside from producing extra carbon dioxide and eventually ruining the beer, is this harmful when it proceeds over short periods?
It depends on how much sugar is present in the bottled beer.

If the beer is bottle-conditioned, a small amount of sugar is added deliberately in order to carbonate the beer. Once the yeast have eaten all the sugar, they stop producing carbon dioxide and fall to the bottom of the bottle. This is not physiologically harmful in any way, but the beer might need to be poured carefully to avoid too much yeast in the glass.

(If the beer is force-carbonated at the brewery, the yeast may contribute to the flavor of the beer, but does not produce any CO2.)

Harpoon's UFO appears to be bottle-conditioned, meaning the carbonation is produced naturally by the yeast. It's a hefeweizen, where the yeast character is actually desired - most people recommend to pour half the bottle, swirl it to suspend the remaining yeast, and then pour the rest.

(Now, home-brewed beer can be harmful if too much sugar is added at bottling - the bottles can explode and cause injury. The beer itself doesn't become harmful, though.)