tvl4free, is this it:

From www.rulesoftheair.com

"United Airlines
Rule 100C
Use of coupons from two or more tickets issued at round trip fares for the purpose of circumventing applicable tariff rules (such as advance purchase / minimum stay requirement) is not permitted. United agents and authorized travel agents are prohibited from issuing tickets, commonly referred to as "back to back ticketing", under such circumstances when there is obvious intent to abuse and/or misuse restricted round trip fares. Agents found issuing such tickets may be liable for the difference between the fare paid and the fare for transportation used. United Airlines has the right to deny transportation to passengers found utilizing tickets in this manner unless the difference between the fare paid and the fare for the transportation used is collected."
Rule 100 C / 22nd revised page UA-7 / May 9, 1996