Oh! Wait...actually...probably the biggest upset in all of Kingdom Hearts history, more than Mysterious Figure could ever hope to be.
In Halloweentown in KH1, as soon as you entered Oogie's Manor there was a Red Trinity Mark on the door. Me, thinking I could come back and get it later, went on to defeat Oogie and his Manor. The problem is, after you beat him, the house collapses with the mark inside. Sure, the Mythril inside the chest behind the trinity mark could be gotten below the manor afterwards...but your journal would NEVER be 100% complete. Why you ask? Because in your journal...the spot where that Red Trinity mark was supposed to be crossed off, it wouldn't because technically you never used it. Sure you got the item which is what is important, but NO! Your journal is screwed over!!!! STUPID SE... -__-