Fix dropped stitch once cast off

A strange one this. Finished (and cast off) Ribbing on my project, only to find I had a stitch that I had dropped and not realised ! Is it possible to fix after the fact? Thanks

If the dropped stitch is near the cast off you can thread a strand of yarn through it and then secure the stitch to a stitch just under the cast off. That’ll work.
If the dropped stitch is farther back you can use this technique to ladder up and then fasten off.