(Thompson @ Jun. 21 2011,07:26)
Nothing bad will happen, but hydrogen peroxide breaks down over time into water and oxygen gas, so it's possible that your old open bottles are no longer as concentrated as they were.
Thanks for the info. Jim's post about freezing makes me think - any better way to store than under the counter?
As an aside, I've been trying to keep my eye out for what looks like recently stocked bleach since reading your post about the experiment yielding unexpected results.