This was mine: I decided to use one statistic, *just* one, in my RE exam, and it was wrong: 'One in two marriages end in divorce', I wrote confidently.
When I left the exam, I recounted the sentence to my friend, it being the only true fact I'd made in the whole exam...I was, in a modest way, proud...
...Only to be corrected to say it was "one in three marriages" that end in divorce. I looked at him. He looked at me. We both burst out laughing. I felt comically crushed.
Then, I ended up getting a B on results day. Apparently the examiner didn't know either.
What's the stupidest thing you did in a GCSE exam you took recently?