Can I retrieve missing Bitcoins that failed to process as Wallet is corrupt? I have been using an old wallet until last year - Bitcoin Version v0.8.5-beta. QT Version 4.8.3. 

I stopped using it as it has become corrupt. I had made several transactions that were not successful before I realized it was corrupt. They are showing in my transactions list as a question mark with 0 confirmations, however the transaction amount has been deducted from my wallet total.

I backed up the wallet.dat file after each transaction and overwrote the previous backup when I did this.

Can I retrieve the missing bitcoins?
If you know the password to your wallet.dat, you should try to retrieve the private key adresses and import them in a wallet such as Electrum, and see if that fixes your issue.

This is a bit of a hassle when you have lots of changeadresses though, but it is better then not being able to access your funds.

When you unlocked your wallet.dat in bitcoin core, simply go to the console and

Use 


  listreceivedbyaddress 0 true


OR, if you have generated new change adresses


  bitcoind listaddressgroupings


To get a list of all the adresses that you are currently using / have balance.

Then,


  walletpassphrase "your walletpassphrase here" 600
  dumpprivkey [your Bitcoin address here]


OR, you could dump all the private keys at once, if you were using alot of different adresses using the following command:


  dumpwallet  


And import these keys in a wallet such as electrum.

See if this resolves your issue.