The two industries that rely on rebates the most are the electronics and software industries. These are also the industries that register the most complaints.
Microsoft, for example, offered a $300 rebate for people upgrading to new versions of Visual Studio or Visual Basic several years ago. The offer, however, was contingent on including the UPC code from the original program, not the newly purchased upgrade; this caught many buyers by surprise.
Other software suppliers, like Symantec and Pinnacle Systems, have been accused of designing rebate models that are over-complicated, require forms to be sent to two or three different processing centers, or are explained in extremely small print. Many software rebates advertise programs at a "net-zero" cost to the customer, and this strategy necessarily involves efforts from the manufacturer, the retailer, and the rebate processor. As a result, the process can become confusing.
Long term ISP contracts are often associated with rebates, as well. At big box stores like Best Buy and CompUSA, personal computers are regularly sold with sizable rebates attached, making the advertised price more attractive to buyers. It is common, though, for these rebates to be conditional upon signing a long term contract with a particular ISP.
Hardware manufacturers have come under fire, also. Dell, for one, has been the subject of rebate complaints involving misprinted receipts, confusing expiry dates, and service representatives who are slow to react. Rebate issues began to clog Dell's customer service forums, leading the company to shut down that portion of the website, and refocus its energy on new online customer care solutions.
Cell phone service companies, including major players like Verizon Wireless and Cingular, as well as third-party retailers like Radio Shack, Wirefly, LetsTalk.com, Simply Wireless, and others have received growing attention due to complex rebate redemption rules. Both carriers and retailers make customers submit rebate claims during a 30-day window, often 6 months after cell phone activation. Some authorized dealers have responded by trying to make rebate requirements more transparent, explaining that the carrier will withdraw payment from them if a customer quits service before the end of the contract.
OnRebate.com, which is most often spotted handling rebates for Tigerdirect.com, is widely known to simply reject rebates without further comment, and without a documented appeals process. For example, many OnRebate rebates require online registration which generates a confirmation email - this email is often not sent, and customer support is lacking.
Sources:
http://en.wikipedia.org/wiki/Rebate_%28marketing%29
http://www.rebateplace.com/
http://www.ftc.gov/bcp/conline/pubs/alerts/rebatealrt.htm