Not so much prop trading but scary/funny none the less...
I do know of the story of someone who bought a lot of option volatility in a stock, only because they read the wrong numbers and mixed the codes up.
I think it was Royal Bank of Scotland v Bank of Scotland.
They looked at the historical volatility for one and thought the implieds were incredibly cheap - mixing up the two stocks
They realised their mistake the next day and rather than minimise the damage and cutting or managing it via trading the gamma, they decided to 'try and corner the market' - it did not go well but I dont know of the numbers but these guys liked to swing big.
...............
I have also sat opposite some guys running a small hedge fund ($15m) - they had no real strategy but were very good sales guys. We shared offices.
One day they had all 4 positions they had on go their way and they made about 4% on the day.
They didn't know what to do - do we run it, cut it, get bigger.....?
It was hilarious to watch.
I suggested to them -
Q: how much are you trying to make each year -
A:10-12%
Q: Its now early March and if you exited all 4 positions you will be up 7.5% for the year.
A: right.
Q: could you easily exit the positions today, sleep on it and then put the same positions back on again tomorrow?
A: yes
Q: then should you do that?
....after 30 mins of discussions back and forth between them they cut their book and locked it all in.
The next day it would have reversed and gave back their gains, and the best part was they were patting themselves on the back for how genius they were. (and you did not need to be a genius to ask them the questions I asked them)
Needless to say their funds did not thrive - at last look I saw one of the guys recent fund was down 30% since 2010.....as I said good sales guys.