Quote from metameta:
If you look at his previous posts on the forum about portfolio margin he actually mentioned that he regularly received margin calls on thursday and fridays. I was suprised by the screen shots in the past of his put sales of 100 contract lots over and over. That kind of concentration is penalized by IB with portfolio margin. It sometimes doesn't show up until the next day. I like it because when you keep selling too many puts on a single issue like even Walmart, the projected margin increase gets larger and larger and is a warning to me to just stop. Watching this makes me never want to short an individual stock, I cringe enough shorting SPY even while hedging my portfolio. I couldn't imagine being short over $800,000 of one security, who knows Microsoft could be crazy and bid again but the SPY doesn't typically crash up (fiscal cliff resolution might make it though).