Common belief: If a contract goes down, you shouldn't bid it.
Verdict: False. We don't bid contracts we expect to make every time.
Another question I get asked frequently: "How do we stay out of 4?" You had 30 high card points, a 9-card fit--you did well to stay out of slam! Sometimes good games will go down because of bad luck: two finesses offside, a bad split somewhere, a surprising ruff.
Sometimes, you'll have the strength for game and it will go down. Was there anything that could be done? No.
One thing that's important to note is whether you are in a pushy game (one that was only bid at a handful of tables) or a game that my grandmother would bid. If you bid a lot of pushy games and they go down, then it's time to consider whether to adjust your style or evaluation. If you bid one of those games that everyone in the room is in and it goes down, then that's just normal.
What percent of the time should you go down in a freely bid game or slam? This depends a little on the form of scoring. You want to bid games more at imps. That said, going down on about 30% of your bid games at matchpoints is normal.
This misconception is a sister to the idea that "double dummy" showing a game could make means it should be bid. Just as with that misconception, you can avoid the problem by looking to see the results of other tables in play. Sometimes you'll see that everyone else made the contract (must have been the excellent defense against you), but frequently you'll see others with the same result.