[MarMema]

Although Russia cut off supplies only to Ukraine, a number of European countries saw a drop in their supplies as well.[3] The European Commissioner for Energy Andris Piebalgs and several affected member states warned that blocking of gas deliveries was unacceptable.[citation needed] Pascal Lamy, director general of the World Trade Organisation, expressed the opinion that all Post-Soviet states should pay market prices for their energy needs in order to improve the efficiency of their economies

"On 28 December 2009, the Slovakian government announced that Russia warned it would stop oil supplies to Slovakia, Hungary, and the Czech Republic over a transit price dispute with Ukraine."

Always these disputes in the middle of winter, of course.

[DesertRhino]

Ukraine refused to pay market price and wanted a heavily subsidized below market rate. (not very conservative)
The pipeline goes thru Ukraine to Eastern Europe.

How exactly do you cut off Ukraine for non payment without cutting off the flow further downline? And if you do not cut off Ukraine, how do you ever get them to pay?
Ask your local electric company why they don’t abandon disconnecting customers who refuse to pay, and instead try to collect with no threat of loss of power?

Gas Billionaire Julia Tymoshenko was the empress of Ukraine then.