There are many stumbling blocks that may prevent you from closing on schedule.  Some, you may be able to control; others, are just opportunities in disguise.  For one property, a huge tree fell through the roof the day before closing, due to a hurricane.  Closing was delayed, but the buyer got a home with a new roof and other new improvements.  Even though complications can occur in the transition from contract to closing, the closing date should not be taken lightly.  Any buyer-related closing delays puts the buyer in default for not meeting contractual obligations, by the date specified in the contract.  When this is done, the seller can terminate the contract. 

In general, a seller will wait to see if the current contract will eventually close, as entering into a new contract or moving forward with a back-up offer may have its own set of risks and drawbacks.  Instead of assuming that the seller will patiently wait for you to close, be proactive.  If you know closing will be delayed, have your agent create a contract addendum to extend the closing date to a new date that you can likely meet.  The seller is not obligated to sign this new addendum, but, is more likely to, if kept abreast of any pressing issues.  Tempers are least likely to flair, when everyone receives frequent progress reports.  Setting a new contract date does not mean that you have to give up hope on closing sooner.  Just make sure everyone is informed that the target closing date should be sooner than what is noted on the addendum. 

For more information on the legal implications of not closing on schedule, consult with an attorney.