The issue was noticed with Virtual Services using REST data protocol handler during load tests with a large volume of transactions and capacity higher than 1.
This causes the VS to return an unexpected response for a specific transaction.
The request is valid and a match is expected, but the response returned is from a different request.
There is some conflicting thread issue that is causing the arguments to not match as expected.
DevTest on releases 10.0 and 10.1.
We may have a patch to address your issue. Please open a support ticket and reference DE296235 and DE307223.
We will be happy to provide you the fix if it matches your issue.