If VPLS STP redundancy is not configured on the network shown in the following figure and the S2-R1 link fails, S1 unblocks S1-R1 and S2 flushes its FDB on the S2-R1 port.
The VPLS core nodes C3 and C4 are unaware of this change and continue to forward any traffic destined for R1 to C2. C2 continues to forward traffic towards S2. This results in data loss because S2 is not able to reach R1 over the customer network.
Depending on the type of data traffic from R1, the latter scenario might not occur quickly.