2 cruises ago, I forgot my suit for formal night. I put the suit on the bed thinking the wife was going to put it into the garment bag...and she thought I already did so. I did bring slacks, nice shirt and ties for other nights so that had to do for formal.
Last cruise, my wife got the packet of passports for the family from the bank safe but didn't check to see that hers was not in it. Luckily, we live 30 minutes from the port and after 3 trips home turning the house upside down to see what legal documents she can find that the Purser would accept...we were the very last passenger to board the ship. The port employees were nice enough to keep the door and 1 x-ray/metal detector open to process her thru.
Most items you forget can be purchased on the ship or at ports but NEVER FORGET LEGAL PAPERWORKS!!!!