You choose 6NT to protect HK and get a club lead.
You dream about 4 spade tricks and think "test spades then play for HA onside" then realise that's not so great. If East has the long spade plus HA, you're dead. So it's 3-3 spades (36%) plus ¼ of the rest (West has the spade guard & East the HA) for about 55%.
So you focus instead on the S9. If SJ or 10 appears from West on the second spade, you can take restricted choice by finessing the nine. No time to work out the odds but that seems a bit better.
Pleased with yourself, you think "SK, SA, then hook if West plays an honour, else find spades 3-3, or play a heart up."
So you play SK. Oops. You've missed the extra 3% chance of East having all six spades and turned +990 into -50 for a 22 IMP swing. Sigh.