I agree with Edith, working with a REALTOR(R) will help reduce the risks of problems during escrow. But to answer your question, here are the most common problems I've experienced in escrow:
1) Seller did not have inspections done prior to listing the property for sale and now discovers termite damage, plumbing problems, etc. that must be revealed in disclosure documents. Many buyers see these disclosures and pass on the property because they believe the problems noted may either be too clostly to repair or delay an escrow closing.
2) Poor mortgage qualification. Sellers should screen buyers in detail regarding their mortgage pre-approval, requiring proper pre-approval documents and proof of funds for down payment.
3) Defects in title. I work with a lot of listings that involve property that has been owned by a family for many generations. Modern title insurance companies do a fantastic job of researching and recording changes in title, and properties that have changed title in the last 10 - 20 years are usually well documented and correct. Properties that were last sold more than 80 years ago often have title problems and require significant research, and sometimes court decisions, to confirm title to the level of quality needed for a title company to issue a title insurance policy. I have a property in escrow that was last transferred in 1908 - it took three revisions to get the title report right. Title problems often take many months to correct; my record for the longest time to correct a title problem was 6 months for a condo that was built in Palo Alto in 1988, and it did require a cour-directed title correction.
I hope this information is helpful for prospective sellers.