A biscuit jointer is a great tool for making accurate wood joints quickly and efficiently. From time to time you can end up with a misplaced slot that won't line up with the mating side. The solution to this problem is to glue a biscuit into the slot, let it dry, and then use a flush trim handsaw to trim the biscuit flush. This will fill in the slot and give you a starting point to recut the slot in the new location.
Fixing an Misplaced Exposed Slot
If you cut the biscuit on the exposed side of the board, you can use a scrap piece of the same wood to fill in the slot. Simply plane the scrap wood to the thickness of the biscuit. Then trace the shape of the biscuit onto the wood and cut it out to make a patch. You can then glue the patch into the slot. When it dries, cut the patch flush with the board using a flush trim saw.