Here is a really common bidding sequence. You will see similar examples in every tournament. I'm putting it up here because I have seen it lots of times from the people I mentor.
The error my friends make is that the 3 spades could be anything from a dead minimum (eg non-vul/vul in matchpoints and you want to compete even if you are going off 1) to a good 17 or 18 hcps inviting game. For some reason there seems to be a reflex from almost every bridge player that compels them to compete to 3 in these situations.
A common way to play here is that a direct 3 is just competitive and if you want to invite game you have to double (ie double is not penalty but an invite to 4). But my friends don't play this way (as most under 50 players dont)...
So here's the sort of typical messes that happen and the thoughts that happen...
1. 1 - pass - 2* - 3 3** - pass - 4*** * - constructive - 8-10hcps
** - I'll bid 3 - I might be minimum but it's matchpoints and my partner is not aggressive enough and we are non-vulnerable
*** - ah - my partner is inviting game - I have a maximum for my previous bid - I must bid game - my partner says I'm not aggressive enough.
And at post-mortem
"Why did you bid 3 spades on that mimimum?'
"I was just competing..."
** - I have a good 15 and good shape - I'd like to invite but my partner will pass - so I'll just bid game..result off 1
Post-mortem
"Why didn't you make a game try?"
"We haven't talked about that?"
Pass** - P - Pass***
* Constructive 8-10
** I am minimum - I should pass
*** Hmmm - Im flat but I have a 4th trump - under the law I should compete to 3 but my partner must be subminimum not to compete
3 spades made
Post-mortem -
"I thought if you had anything you'd have bid 3 spades"
"I can't - I'm minimum - you have to"
The major problem is the lack of defined rules about
a. what bidding at the 3 level means in competition
b. how to invite game
c. who should compete
d. when pass is the better option...
So, it's worth having agreements about what a 3 level bid promises in a sequence like
because for some reason most people can't help themselves bidding 3 on 12 hcps or 17 hcps...