This is a good question, and I hope someone can answer it for us. This has happened several times to me. I trade spreads, and on occasion I look at my portfolio and see that a position has changed - sometimes the number of contracts stays the same with different symbols, sometimes the number of contracts changes and has different symbols, and in one case the number of contracts changed with new symbols AND I suddendly had 20 shares of some stock I never heard of. But I never lost any money on these surprises, and none of the characteristics of the positions seem to have changed that I could tell.