Murphy says: “Anything that can go wrong, will go wrong”. When it happens to your store it’s best to get the information about it ASAP and present the error in the most humane possible way. Let’s do it!
Since ever Magento presented it’s errors on the front-end for everyone to see and it was good for the developers but once the site has gone live that couldn’t be an option anymore. In Magento 1.4 those errors are now hidden and all your customer could get about the actual error was the “error number”.