EW do not come any further than 1NT, whereas 3NT is an excellent contract.

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

'I thought you showed 12-14 points', says west.
'Would I double then without shortness in diamonds?' east replies.
'You could have ♦Ax or ♦Kx or ♦AQ', says west.
Who made the mistake?

Solution

West made the mistake.
True, east could have doubled on 12-14 points and something like ♦Ax or ♦Kx or ♦AQ. But with such a hand he would have passed west's 1♠ bid!
Doubling first and bidding no trump in the second round, like east does, shows 18-19 HCP. A hand too strong therefore for a direct 1NT overcall. The idea behind it is completely logical: west can bid 1♠ on zero points, a 1NT rebid by east on a mere 12-14 would therefore be suicidal.
This principle applies in many bidding situations: if a player is too strong for a direct bid, he doubles first and does the bid in question in the next round.
Another example: