As soon as I made and equipped the slingshot, despite being listed as separate commands on the menu, pressing "punch" also fired the slingshot. Reassigning the commands did not seem to affect this.
Played this game tonight on Win10 using keyboard & mouse and, as others have said, I found it hard to pick up the mushrooms. Even running over them tapping the space bar wouldn't get them sometimes, other times I picked them up right away.
