Here is the link to the thread, where the patch addressing malleability was discussed:
https://bitcointalk.org/index.php?topic=8392.0The whole MtGox goxing was only possible because:
1) They were issuing transactions with sloppy signature format
2) This was accepted by the network for some time
3) New bitcoin client with tightened rules was released
4) Their sloppy transactions started being rejected
5) Exploiters "fixed" those transactions
6) MtGox sloppy software didn't notice transaction went through
7) Mt-gox sloppy software didn't notice output's were spent (making them unaware that they lose coins). It didn't even lock outputs which are used in pending transactions!!

MtGox incompetent customer support resubmitted transactions manually without looking into issue and alarming developers. Or maybe they did, but developers were too busy/confident/not able to fix the problem.
Overall: MtGox are incompetent bunch of liers
To be fair, the "workaround" implemented by Gox following a core dev recommendation, was horrible. Staging transactions is much worse than the initial "bad sig" problem, IMHO.