Playing in a team match online (as a sub), I land up in 3NT. West leads the Jack of hearts.
Thinking hazily, I figured that West would lead Jack of Heart from one of two holdings: Jack-high and KJ10. If he's leading from KJ10, I should cover and if he is leading from Jack-high, I should wait. I thought it was 50-50. I covered and it was too late, since East took his King heart and drove out my Ace.
My analysis at the table was wrong, of course. There are more ways for West to have Jack-high hearts than specifically KJ10. Moreover, the opponents have 7 hearts and they're likely to be split 5-2 or 4-3. Holding up the queen could either freeze the suit (if east has the king and 5 hearts) or drop the king (if either player has king-doubleton).