Ppruners:
This phenomenon is referred to as "Sporadic E" (go to:
http://en.wikipedia.org/wiki/Sporadic_E_propagation for more info). During Sporadic E conditions VHF and UHF radio range is increased significantly due to some of the signal being bounced off a very small cloud of heavily ionized atmospheric gas. In other words the VHF signal is temporarily acting like a HF signal.
There have been recorded occasions in the past where this phenomenon has caused problems for ATS in Australia. On one occasion (lasting 3 days) in 1975 on one of the old Port Hedland FIA frequencies (I think it was 120.7), FSO's and pilots were sharing the frequency with Dili Tower in Timor! The Port Hedland FSOs and Australian pilots were even able to talk directly to the Dili tower controllers on VHF 120.7, and did so. Another occasion in 1979 involved Sydney FS sharing a VHF frequency with Auckland Control and also being able to talk directly to the NZ controller on VHF.