As per the Point of Taxation Rules, 2011, service tax liability even arises on accrual basis if any of the criteria specified in the rules are fulfilled. In such a case the tax would be remitted to the Government and later on claimed from the service recipient.
Secondly, a service provider, who is an exporter may first pay service tax on his output service and then claim rebate. Sometimes such exporters are required to pay tax first to their input service providers and then claim it as refund from the Government.
In all probability you would have encountered such scenarios.