This is board 1 from the BBO Forums Sunday Daylong. At trick 11, the position (with South to lead) was:
East knows that I (South) still hold ♣3. So when I cash my ♥J winner and discard dummy's ♦Q, a club discard by East instead of the worthless ♦5 is shocking; there is no other way to describe it.
The only South hand where it doesn't make a difference is if South had a 18 HCP hand 1NT and his last two cards are ♠Q ♣3. In all other situations, a club discard is simply nuts.