Jeff,
In my experience, test kits start to go bad before the expiration date. The moment you open the reagent bottles, they begin to change:
1) Water evaporates from liquid reagents, effectively strengthening their concentration.
2) Carbon dioxide from the atmosphere dissolves in the reagents, altering their pH. It may also directly interact with acids and bases, causing the formation of water and weakening of the reagent.
3) Powdered reagents absorb water from the atmosphere and depending on how reactive they are may undergo early chemical reactions that reduce their strength when used in a test.
4) Finally, if your reagents are exposed to heat or light, they may break down that way.
So even more important than the expiration date is how long your reagents bottles have been open and the conditions they've been stored in. If your kit has been open the entire time you've owned it AND it's expired, you can pretty much assume it's not going to be accurate anymore.
Hope this helps,
Eric