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.
{
    AssertLockHeld(cs_main);
    return status.cur_num_blocks != chainActive.Height();
}
QString TransactionRecord::getTxID() const
{
    return QString::fromStdString(hash.ToString());
}
int TransactionRecord::getOutputIndex() const
{
    return idx;
}