If your infant website has downtime, people will close the tab on their browser and in all likelihood never return. (Probably the only exception to this is if you're receiving sustained media attention, much like Pinterest is now.)
If your new pet store runs out of stock, those bothering to try you out will think I should have known to themselves and go back to the established competitor just a few blocks away from you.
If your newly opened tax consultancy cannot submit a client's tax return on time - because too late down the road you realise this client forms part of a complex group structure that you don't have the expertise to deal with - then next year you are losing that client.
The marketplace is very unforgiving. People pay money for something because it's usually something they don't have the time/resources to make or do themselves. Attention spans and patience are at an all time low, while expectations and stress levels are at near highs. "Sorry" doesn't make the bitterness of non-delivery taste better. And with most people already overloaded and stressed out from life in general, they aren't going to display the forgiveness to try you again.
I've learnt from personal experience that getting people (yes, even those you personally know) to visit a website just once is incredibly difficult, so if they do go to your site and the message of Error 404 comes up, they will not waste precious seconds of their time trying again, when there are millions of other sites to visit.
When you open doors it's best to start with a simple but complete offering. Don't offer more than you can deliver. If it's simple you can test everything and ensure it works. Simplicity also allows you to quickly identify and rectify an error or malfunction. Then as you expand and have the kinks ironed out, you can start adding complexity - in manageable steps.
Nobody cuts you slack just because you're a new player.
You have it all to prove and each customer only gives you one chance to prove it.