1. Select the subject that was not accurately cut-out
2. Slightly expand the selection by going to >select >modify >expand by some pixels, so that the selection includes the inaccuracies
3. With the selection mode set to subtract, select the subject or object again and this time contract rather than expand by a few pixels

The resulting selection includes the inaccuracies surrounding the cut out subject and slightly overlaps the subject

4. Hit generative fill and let it do its magic

First seen in a Video Tutorial shared by the Wade McMaster (go to time stamp 1:49)