More often than I'd like to admit, when I am cutting biscuits I mess up one of the slots. This usually happens when I have a board upside down, so the heights of the biscuit slots no longer match up. If the slots are far enough off, it's no big deal since you can just cut a new slot. But if the new slot overlaps with the mis-cut slot, this won't work since your biscuit slot will then be too big and the biscuit will lose most of it's strength.

If this happens to you, I've found that this is easily fixed by gluing a biscuit into the incorrectly cut slot and then putting in a biscuit. What for the glue to dry then use a trim saw to cut off the half of the biscuit that is sticking out. You then have a smooth surface into which you can cut a new slot.