Took a reading this morning with mine & noticed it finished better than expected. Pulled the hydrometer out of the sample tube and saw the wort sitting on top of the lead weight. Glass chipped off around the bottom of the weight.
I've busted 2 hydrometers and 2 thermometers. I just broke my thermometer this weekend and haven't bought a new one, but I think I'm just going to get a bi-metal 12" from MoreBeer. Hopefully I won't break that type.
Mine developed a tiny crack that let water in and messed up the readings. It made me think my gravity was 20 points wrong, so I overcompensated and messed up a batch before I realized that my hydrometer was reading -20 points on plain water.
Test your hydrometer with plain water before you go believing it.