When Frenchy takes Capt Morgan to the cellar to remove a bullet, Frenchy unlocks the cellar door and promptly relocks it after they enter the cellar. A few minutes later, Slim comes down the stairs in the cellar with the first aid kit and the hot water. Frenchy has been in the room with the wounded man all this time and couldn't have unlocked the cellar door.
The scene showing the patrol boat firing at Morgan's boat, the machine gun is not recoiling, the bolt is not moving nor is there any gun smoke. This type of MG the bolt would recoil after every fired cartridge and would cause the whole gun to have some recoil effect. The gunner was just standing there, he would have to move his fingers or thumbs to fire the gun.
When Harry tells Eddie he can relax after picking up the passengers, Eddie cycles the lever action on his rifle, which ejects the shell that was chambered. Harry then tells Eddie not to unload because they aren't home yet. But just cycling the lever action won't unload the rifle unless the chambered round is the last one, and it would be very unlikely in the situation for Eddie to be carrying a rifle with only one round and for Harry to know it only had one round in it. That would defeat the purpose of carrying a lever-action rifle. And in fact, a short time later, they run into a patrol boat and Harry fires several shots with the rifle with no further cycling of the lever action between Eddie's cycling of the action and Harry firing the first shot, proving it wasn't unloaded.
When Harry calculates how much Mr. Johnson, his fishing customer, owes him, he says more than once that it is 16 days x $35/day, + $275 for the rod and reel he lost. That totals $835. When he has Johnson's wallet he says Johnson owed him $825 (more than once). Harry calculates the amount and says, "Now you got a little credit so that'll be 825". The difference between 835 and 825 is, presumably, "...a little credit".