If your project is behind schedule but it's going to be finished eventually, the best thing you can do is to simply let people know. This schedule slippage can happen for any number of reasons. If, for example, you're developing a mobile accessory, then the fact that Apple changed the iPhone's connector will throw you off schedule. Generally backers are understanding about delays...as long as you explain yourself in detail.
If you have a delay, update your Kickstarter page to explain why and, crucially, how lengthy you expect the delay to be. You simply cannot provide too much information to backers, who begin to get nervous the day after your Estimated Delivery Date has passed. That date, by the way, is now required on all projects. Updates on a near-daily basis aren't a bad idea if you want to keep your inbox from overflowing with hate mail and legal threats.
6. Problem: Your project didn't work out
You don't need to search for long to find tales of Kickstarter backers upset that they made funding donations and never received their rewards, or even any information about what went wrong.
This isn't entirely unexpected. Kickstarter may require an Estimated Delivery Date on all projects, but these are still estimates, and there's no enforcement of or consequences for missing that date. (Kickstarter says it is the "creator's responsibility to complete their project," but also, "Kickstarter does not guarantee projects or investigate a creator's ability to complete their project.") As a result, backers are becoming increasingly nervous about funding projects if they aren't dead-certain the projects are going to pan out eventually.