In the transactions list, if you right click a transaction and choose "view transaction details", the "Output index" listed in the resulting window is usually wrong.
With outgoing transactions, if you have a transaction with outputs like:
vout 0: AddrA
vout 1: AddrB, which belongs to sender
vout 2: AddrC
vout 3: <change>
vout 4: AddrD
Then the output index as reported by the GUI will be:
tx to AddrA: 0
tx to AddrB: <transaction not listed>
tx to AddrC: 1
tx to AddrD: 2
With received transactions, the output index always seems to be reported as 0 in transactions I've checked.
I'd guess that rather than the actual vout index, some internal wallet index is being reported.
This is 0.13.2. I don't know when the bug was introduced.