When using Lift and Drop to move boxes, if I hit Backspace to undo while the Sokoban is pushing the box, the Sokoban returns to his previous position but an invisible ball is located at the drop point. If I then use Sokoban to move this box, it appears.
I can then complete the level using this box and have an extra box left over.
When I play my solution, the extra box remains invisible, even when pushed, and the hole it occupies does not appear filled. But when the whole solution is played out, an extra box remains.
Email with two examples sent separately