Source: Mr Bridge
Same players like conventions so much that they use them on any hand, however inappropriate. Here is a novel way to lose points:
‘The unusual 2NT – both minors – but it should be at least 515 in ‘he two suits. Take a moment to appreciate just how awful a bid this is:
• Neither of North’s suits is good;
• One of the suits is too short;
• With equal length in the minors, partner will pick the wrong one;
• North has a lot of defence against a spade contract and so does not want to encourage a save.
So enamoured was North with the unusua12NT overcall that he perpetrated it with only 5/4 in the minors. South
took the obvious save (needing only to be opposite something like 
A-K-x-x-x, 
A-]-x-x-x and a singleton heart for
the ‘save’ to make). The result was 1,100, against a game that was not ironclad.
Do not blame the convention for this outcome. Conventions are like guns, blameless in themselves – it is all about whose finger is on the trigger. Here is another lovely use (should I say abuse) of conventional bidding:
1How good is your 12-14 opening? 2Minimum, 12 or poor 13
North could raise 1NT to 3NT but was unable to resist asking first whether the opener was maximum or minimum. West led the 10, covered by the J, K and A. Declarer crossed to dummy in hearts and ran the Q, losing to
West’s K.
Since declarer was marked with the A-Q and appeared to have the A, West realized that he could not also have the 
A, as that would give him a maximum for his 12-14 INT. Thus, West switched to the K and then led the J, defeating the game. If North had simply raised to 3NT, the best defence would have been much less clear.
Here is an even worse form of abuse:
1 landy, the majors
West overcalled 2, either forgetting the agreement that this showed the majors, or choosing to ignore the fact. North
swung the axe and the carnage began. 1,400 later (I kid you not), West still thought his 2 bid was ‘unlucky’. It is not only poor bidding to bid as West did here; it can cause ethical problems too. If an explanation, alert or the like tells you that there has been a misunderstanding, you have a duty to ignore it. Arguably, West should have passed 2
as that is what he would have done if East had said 2 was natural.
Summary
It is unfair to blame a convention if you use it on the wrong hand. Play only those conventions you need, discuss them in detail with your partner and then use them only on the right hands. Otherwise, conventions are not an asset, they are a liability. Never use a conventional bid ‘because it is there.