After you've run a replay and see errors in the Errors Tab, then what?
You first want to right-click on the error, and see the list of comparison options. Note that on some computers the options will be grayed out. This is a bug, and those options still work.
When you select a Transaction Compare option, the values from the current replay or load test will be compared against the original recording.
Use the Request Headers/Content compare to see what parameters were sent to the back-end on a POST.
Use the Response comparisons to compare the web page returned from the server with the original.
The Response Headers/Content have really cool head's up display to drill down into the HTML and show you exactly what changed.
The Visual Compare is a dual heads-up side-by-side visual rendering of the page.
The most common causes of playback errors are:
1. Workflow errors, i.e. inputing the wrong password, or having the user do something impossible like click on the 10th item in a search when the search returned nothing.
2. Configuration errors of fields that were hand-coded to do something special via javascript. These will have to be configured by hand in the test case, and is a normal part of test case development.