Hi, joefish...
Generally, the primary concern with OD is with the zeostart additive...an OD in the beginning can cause very quick drops in phosphates/nitrates and can cause some tissue loss, etc. I myself OD'd zeostart when I started (10x what I was supposed to use!) but didn't personally experience any severe reaction.
Zeolite, 'bac and 'food dosings are generally more forgiving...it's not like a 10% OD would cause huge problems, and in fact there is a decent range in what people use/recommend. The health and color of the corals is a pretty good indication of any sort of OD.
In addition to the above additives ('bac, 'food and 'start), there are a few others that some use once the tank is zeo-established (3+ months in) to get better coloring, etc.; an iron additive, an amino acid one, and an iodine-related one, for example. In particular, there is one other additive, Zeospur2, that can cause a lot of problems if it is OD'd. It is sometimes used by people without the rest of the system, because it can have a dramatic brightening effect on SPS even in non-zeo tanks, but if OD it can cause STN, etc. However, once a tank is fully zeo-established, its use can cause a lot of growth and really nice coloring. It is added once every two weeks or so, in a dosing similar to zeostart (1ml per 100g) or less. Since it is purely optional (along with the iron, iodine, etc.), a lot of folks don't use it at all...I didn't start using it until a month ago, in low quantities (and it did cause a growth spurt, btw).
Arthur