yes Shaik i also thinking same.....
I am calling plugin on 'win' message and tried to update opportunity description only.
Its very simple thing i want to do....but dont know why getting error.
this is error log i found,please have a look:
<s:Envelope xmlns:s="schemas.xmlsoap.org/.../"><s:Body><s:Fault><faultcode>s:
Client</faultcode><faultstring xml:lang="en-US">Entity Id must be specified for Update</faultstring><detail><OrganizationServiceFault xmlns="schemas.microsoft.com/.../Contracts" xmlns:i="www.w3.org/.../XMLSchema-instance"><ErrorCode>-2147220989</ErrorCode><ErrorDetails xmlns:a="schemas.datacontract.org/.../System.Collections.Generic"><KeyValuePairOfstringanyType><a:key>ValidationPath</a:key><a:value i:type="b:string" xmlns:b="www.w3.org/.../XMLSchema">/0</a:value></KeyValuePairOfstringanyType></ErrorDetails><Message>Entity Id must be specified for Update</Message><Timestamp>2016-04-04T11:01:16.1232311Z</Timestamp><InnerFault><ErrorCode>-2147220970</ErrorCode><ErrorDetails xmlns:a="schemas.datacontract.org/.../System.Collections.Generic"/><Message>System.ArgumentException: Entity Id must be specified for Update
Parameter name: Target</Message><Timestamp>2016-04-04T11:01:16.1232311Z</Timestamp><InnerFault i:nil="true"/><TraceText i:nil="true"/></InnerFault><TraceText>
[CrmPackageforBhawik.PluginsforBhawik: CrmPackageforBhawik.PluginsforBhawik.PreOpportunityWin]
[9e9ad9ac-52fa-e511-80ce-0050569e09d5: PreOpportunityWin]
Entered CrmPackageforBhawik.PluginsforBhawik.PreOpportunityWin.Execute(), Correlation Id: 70f53635-cce4-42e3-9553-42c67ee70ccc, Initiating User: a0f456a2-9d67-e511-80cd-0050569e09d5
CrmPackageforBhawik.PluginsforBhawik.PreOpportunityWin is firing for Entity: opportunity, Message: Win, Correlation Id: 70f53635-cce4-42e3-9553-42c67ee70ccc, Initiating User: a0f456a2-9d67-e511-80cd-0050569e09d5
Exception: System.ServiceModel.FaultException`1[Microsoft.Xrm.Sdk.OrganizationServiceFault]: Entity Id must be specified for Update (Fault Detail is equal to Microsoft.Xrm.Sdk.OrganizationServiceFault).
Exception: System.ServiceModel.FaultException`1[Microsoft.Xrm.Sdk.OrganizationServiceFault]: Entity Id must be specified for Update (Fault Detail is equal to Microsoft.Xrm.Sdk.OrganizationServiceFault)., Correlation Id: 70f53635-cce4-42e3-9553-42c67ee70ccc, Initiating User: a0f456a2-9d67-e511-80cd-0050569e09d5
Exiting CrmPackageforBhawik.PluginsforBhawik.PreOpportunityWin.Execute(), Correlation Id: 70f53635-cce4-42e3-9553-42c67ee70ccc, Initiating User: a0f456a2-9d67-e511-80cd-0050569e09d5
</TraceText></OrganizationServiceFault></detail></s:Fault></s:Body></s:Envelope>