NEW YORK (CNN/Money) - Ah, expiration Friday. A fine day for a walk in the park, catching an early matinee, maybe catching up on some reading. Anything really, so long as it prevents you from making the mistake of buying and selling stocks.

What is now known as quadruple witching -- the quarterly expiration of stock options and futures and index options and futures -- has always been a recipe for wild volatility, and while things aren't so bad as during the bad old days, it's still bad enough that many seasoned Wall Streeters opt to stay away from the market. You cannot be sure whether what the market is doing is real or artificial, and the action in stocks can be downright squirrely.

For those who can't help themselves but play in the market's currents, Friday's expiration probably won't be particularly dangerous, reckons McMillan Analysis president Larry McMillan. He notes that much of the expiration effect appears to have occurred earlier this week as options players unwound positions in S&P 100 options.

But he warns that if the S&P 100 index, which closed Thursday at 501.76, rises above 510, there's a raft of call options -- which give investors the right to buy at a set price -- that would suddenly be in the money. That would force the investors who sold those calls to scramble to buy them back, and would set stocks higher.

The call sellers very much do not want that to occur, and they will do what they can to prevent it. The call owners very much want it to occur, and they will do what they can to make it happen. So even though it should be safe, the potential for choppy action is very much there.

Individual stocks, too, can see some bizarre action as options buyers and sellers duke it out. The action can get truly heated as the closing bell approaches, and investors try to "pin" stocks at set levels. If you suddenly see your favorite stock jump up or down a buck a couple of minutes before the close, it probably has nothing to do with the news and everything to do with games in the options arena.