He can solve the maths. He cannot give his name.
Introduce a capable character with missing memories by showing what they can do before asking what they know about themselves.
A character takes a warning seriously but protects against the wrong danger. Make the mistaken assumption shape the precautions.
Frankenstein: Chapters 20 and 23, the wedding-night threat and its fulfillment.
After Victor destroys the proposed companion, the creature threatens to be with him on his wedding night. Victor understands the date but assumes he himself is the intended victim.
Victor anticipates his own death and prepares for confrontation. On the wedding night he sends Elizabeth away while searching for his adversary; the attack falls on her instead.
Do not withhold the warning from the reader or make every sensible precaution fail by coincidence. The particular mistaken assumption must explain the exposure.
Chapters 20 and 23, the wedding-night threat and its fulfillment. ↗
Selected passages: Chapters 20 and 23, the wedding-night threat and its fulfillment.
A threat can name a time without naming its target. The frightened listener fills in the missing part, then makes a plan around that guess. To make the mistake matter, connect it to one precaution that protects the assumed target while leaving another person, place or resource exposed.
Write the threat exactly as the protagonist receives it. Underline the facts it supplies and list the assumptions the protagonist adds. Choose one protective action that follows those assumptions. Check what that action leaves less protected, and give the reader enough evidence to reconsider the assumption later.
Use in my storyIntroduce a capable character with missing memories by showing what they can do before asking what they know about themselves.
Build a mystery around sincere witness statements whose ordinary wording leaves out something crucial.
Make an impossible object convincing in a horror mystery by showing how careful observers check one another's accounts.