Good batteries (which are usually rather expensive), including hologram-tagged Nokia batteries, come with a special protection circuit inside. It triggers if the charging voltage goes above 4.3 volts for some reason (say, a failure in the phone's charging control circuit). In addition it protects the battery from a couple of other issues too. Without this extra protection a random failure in the phone charging circuit will make the battery explode violently. In addition there's the packing material, already mentioned. If it gets punctured the battery can easily catch fire.

Stay away from cheap batteries from unknown sources. Some of these can be of OK quality, and meant for radio controlled planes and the like. Those batteries don't have a built-in protection circuit because they are meant to be assembled into aggregates with a separate protection circuit added by the RC amateurs. They are easy to get by, and cheap enough, but don't put one in your phone. Problem is, some unreliable re-sellers may try to make a quick buck by re-packaging these kind of batteries into something that fits a phone.

Oh, and the super-high capacity is a dead give-away of an unreliable seller, because you just can't squeeze more than a certain amount of capacity into the given volume of the N900 battery compartment. Maybe a little bit more than the original 1300 mAh (IIRC), but never 1700.