Passed round to me in 4th seat, I have 15 HCP and a flat hand and down grade to 1NT (trying to be clever perhaps).  Had I opened 1♥, partner may muster up a 1♠ bid and now I bid 2♠ and that is where we play.  This is a much better spot than 1NT - lesson here, bid your hand!!!  I am sure a later diary entry will advocate when to up or downgrade a hand though....!