There’s a lesson for the unwary investor in the chart at right, posted Monday by Paul Santos on Seeking Alpha. It shows what he calls a mid-morning “flash crash” in Apple (AAPL) — a precipitous drop in the stock’s share price with no apparent trigger except a sudden burst of selling.

The final minutes of trading last Wednesday saw a similar pattern — a spike in volume, a drop in share price. Two days later, investors were buzzing about the opposite pattern: a burst of buying and a quick, short-lived pop in Apple’s share price.

What does it mean? It suggests that in the few hours of trading left before Apple unveils its next iPad, the stock is at the mercy of high-frequency trading algorithms that can snap up or dump thousands of shares in the blink of an eye.

Despite the avalanche of advice that may cross your screen — especially on sites like Seeking Alpha — this is not a good time for most investors to try to second-guess the market’s reaction to whatever Apple will or won’t announce on Wednesday.

Paul Santos, who posted the chart above, followed up Tuesday with more charts showing Apple’s share price falling after four of the last seven big product announcements. “Given Apple’s outstanding performance as of late,” he writes, “the odds are significant that once again, the same will happen.”

Santos seems to be inviting readers to use the iPad event as an opportunity to speculate on Apple. We note, however, that he’s not doing it with his own money.