If it is, then it adds a nice feature to travelling to DLR and other
stations without the barriers. I can just go to Canary Wharf shopping
centre, buy some stuff, and go back, paying just 1 quid (i live in
Z2). Is there anything that can go wrong in this method? Any exception
from entering and exiting at the same station etc.?

You'll be charged two maximum fares - one for your outbound journey and one
for your return.