First, the drill rig and equipment obviously can be sources of contamination if they are not properly cleaned from the last site. Also, the mechanical equipment itself may be a source of leaking petroleum products (fuels, lubricants, hydraulic oils, etc.) that may inadvertently come into contact with tools or pipe going into the hole.
Care should be taken as to from where the drilling water or decon water comes. The source of the drilling water should be from a potable supply -- not from a creek, lake or shallow well. Potential contaminants can include volatile organic compounds, as well as bacteria.
Bentonite drilling muds often contain appreciable levels of heavy metals that can appear in water samples if the well is not properly developed of all drilling muds. Ground water with a low pH can dissolve metals from the drilling mud. On the other hand, if a well has an abnormally high pH, it may be due to the lime filtrate around the cement grout placed at the bottom of the casing. Extra development time usually can lower the pH.
Protective locking casings are a favorite place for bees and wasps to build a nest, but do not spray the well head with bug spray; they contain solvents and pesticides.
Lastly, (but I'm sure you know a few more "not to dos") is a practice that is generally considered out of our control -- namely, contamination from the laboratory. Although laboratories have improved their quality control in the last few years, there still seems to be an occasional odd occurrence of acetone, alcohols and other chemicals from the cleaning of laboratory equipment and glassware. If these compounds appear in the data, check with the lab to determine if it is from the lab.
It is easy to contaminate samples when the detection limits (parts per billion or less) are so low that we need to constantly be aware of potential sources. While you're at it, don't store your sample bottles in the trunk with the gas can or generator!
