In reply to UVM_LOVE:
One of the key things is to hold the registers in the DUT and in the shadow synchronous.
With set or randomize from a sequence the content of the shadow changes, but the DUT have then other content. The shadow content will be considered as the desired values. mirror reads the content from RTL registers. This is now the mirrored value. Now we have to make sure both values are the same. To synchronize the register content you have to call update.