♠	8 3			♠	A 10 9 7
♥	K 7			♥	A 8 3 2
♦	K Q J 5			♦	9 3
♣	A K 6 5 4			♣	Q J 3

On a spade lead East easily gathers ten tricks: five clubs, two diamonds, the ♥AK and the ♠A. He loses two spade tricks and the ♦A.
‘I have eleven points’, says East, ‘which I showed by bidding 2NT, 10-11 points. Why don't you raise to 3NT, having 16 points?’
‘Sorry’, says West, 'I thought I had shown these already.’
Who was wrong?

Solution

East was wrong. Probably he did not notice the fact that his partner's 2♦ was a reverse bid; it showed 16 points or more.
Over 2♦ East should bid 3NT therefore; he knows EW have at least 27 points between them.

If one of the partners has opened one in a suit, a later 2NT bid by either of them is invitational to game (unless the bidding has been made game forcing, of course). The 2NT bidder invites to 3NT opposite his partner's known strength. Some examples:

¹ 12-18
² Invitational opposite 12-18, showing 10-11 therefore (West's minimum point count is decisive!)

¹ 6^+
2 Reverse 16⁺; forcing for one round (not game forcing in view of ¹)
³ Invitational opposite 16⁺, showing 6-7(8) therefore; West will pass if he has a minimum (16-17)

¹ 10^+
2 Reverse (15)16⁺; game forcing in view of ^1 
3 Still 10⁺, no upper limit since the bidding is game forcing (a practical agreement is that 2NT shows a single diamond guard - or serious extra strength - and therefore doubt whether 3NT is the best contract; with 10-12 and a double diamond guard East will jump to 3NT)

¹ 12-18
² East's 2♥ bid shows preference only and therefore a weak hand (6-9); often he has a doubleton in hearts only (if 1♥ shows a five card suit, he is certain to have a doubleton only - after all he did not raise hearts at once)
³ Invitational opposite 6-9, showing 17-18 therefore