In this paper we define and explore proof of violation (POV) for cloud storage systems. A POV scheme enables a user or a service provider to produce a precise proof of either the occurrence of the violation of properties or the innocence of the service provider. POV schemes are solutions for obtaining mutual nonrepudiation between users and the service provider in the cloud. Existing solutions for obtaining mutual nonrepudiation only support epoch-based POV. The drawback of an epoch-based POV scheme is that violation of properties can only be detected at the end of an epoch. We propose a real-time POV scheme in which the auditing can be performed at the time of each file operation. To the best of our knowledge, it is the first scheme that can perform real-time POV for cloud storage. In addition, each client device only needs to store a partial hash tree of files in an account. Experimental results are presented that demonstrate the feasibility of the proposed scheme. Service providers of cloud storage can use the proposed scheme to provide a mutual nonrepudiation guarantee in their service-level agreements.