When buying/shorting the first set of shares in a contract, the button sporadically shows double the amount
Say you currently own 0 shares, and want to buy 10. For a short moment after clicking the "Perform trade" button, it will show the text "Buy 20 shares". You will not actually buy 20, but it has been observed, when buying to the limit of your holdings that the system says "not enough funds" on first try, and then let the trade through on the second. Might very well be related.