I've been using the same hydrometer for over a year and never really had an issue with it - I always attributed the slightly-off SG readings I got to my beginner's inexperience with making beer.
So with my current Saison Dupont clone, I wasn't that surprised when my starting gravity read about .015 higher than the recipe said it would. Then when the FG was also reading about .020 higher than the recipe specified, I started thinking (usually a bad idea, I know).
So I threw some tap water in there. At room temp, 70 degrees, the hydrometer read 1.007. Okay, but tap water isn't pure water. So I took some bottled spring water (Poland Spring, to be specific). That, at room temp, came in reading 1.005. Also, I've read that most hydrometers read accurate at 60 degrees, and you add .025 for every 10 degrees higher.
Is my hydrometer reading too high?