EW end up in the wrong game.

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

West is defeated by one trick in 4♥: NS make the ♣AK and the ♦AK (West is lucky to escape a diamond ruff...).
3NT is on ice.

East: 'I thought you showed six hearts.'
West: 'Surely not, I'm showing at least five. You need at least three-card support to raise.'

Who erred in the bidding?

West was wrong. In a sequence where partner's bid hasn't taken away bidding space, opener shows a six-card suit if he rebids his suit. So East was right in raising on a doubleton.
West did show six hearts because:
- With any 5-4 hand he could have bid his four-card suit (clubs or diamonds)
- With any 5-3-3-2 hand he would have rebid 1NT
- So there aren't any distributions possible with which he would rebid 2♥.

Admittedly, the distribution of this deal is very 'cruel' and perhaps a bit made up. After all, most of the time 4♥ will not be such a bad contract. Furthermore, change West's ♦2 into the ♥2: he would be right in rebidding 2♥ then and the partnership would then 'correctly' end up in 4♥. Still that contract would suffer defeat by the same four top tricks as before. And still 3NT would be on ice.
But bidding cannot be exact. We reason as follows: if partners discover that they have eight cards in a major suit between them, it usually is best to play a contract in that suit; if they have seven cards in a major suit between them, it usually is best not to play in that suit (at game level, that is).
However, now and then this turns out to be untrue. Well, that's bridge.
The important thing is that both partners must know whether they have eight or seven hearts between them. Only then they are able to take the decision that is usually (!) winning.

Let's line up some auctions:

¹ Six-card suit, since with 5♣-4♦ South would support diamonds; with 5♣-4♥/♠ he would bid 1♥/♠ and with 5♣-3-3-2 he would bid 1NT.
This situation is similar to the one after the beginning: 1♦-1♥, 1♥-1♠ (our problem case) and 1♠-2♣. Every time, by rebidding his suit the opener shows a six-card suit, since responder's bid hasn't skipped a suit.

But then:

² Five-card clubs possible but only if South has 5♣-4♦ and not enough extra strength to bid 2♦ (reverse). After all, North's response skipped the diamond suit
South will avoid such a 2♣ bid with 5♣-4♦ if ever he can, since often he has another option. With 2-2-4-5 he can often rebid 1NT and with three-card heart support he can consider raising that suit (when considering such 'improvisation bids' he must take a good look at the location of his face cards).
With 5♣-4♥ he simply raises to 2♥, with 5♣-4♠ he simply bids 1♠ and with 5♣-3-3-2 he bids 1NT.
This situation is similar to the one after the beginning: 1♦-1♠, 1♥-2♣ en 1♠-2♦. Every time the response skips one suit, so the opener may rebid his five-card suit (but he will not often do so).

³ Regularly on a five-card suit: South will do so with 5♣-4♦ or 5♣-4♥ if neither 1NT (with 2-4-2-5 or 2-2-4-5) nor 2♠ (with 3-1-4-5) are attractive bids.
Since North's response skips two suits, the chance that South, out of necessity, rebids his five-card suit is considerably bigger than in ².
This situation is similar to the one after the beginning: 1♦-2♣, 1♥-2♦ en 1♠-2♥. Every time the response skips two suits, so the opener will regularly rebid his five-card suit.

To conclude with: don't be too rigid. With:

...and after this start:

...South really does better not to bid 2NT. After all, he lacks guards in both unbid (red) suits. So even though he has 5-3-3-2 he'd best rebid 2♠, not bothering that he shows a six-card suit. If only because a notrump contract (if that's what it's going to be) will be played by the right hand (partner's!) then.
This five-card suit is about as strong as an average six-card suit.