In a move that will have economists rechecking their spreadsheets, the S&P 500 closed above 6,400 for the first time, proving that even numbers enjoy a little adrenaline.

Market watchers caution that the milestone may be less a triumph and more a reminder that highs attract every kind of investor, including the ones who bring snacks and questionable life choices.

Reason for caution #1: The rally has been powered by a small cluster of tech giants, making the breadth of the move about as wide as a one-seat airplane.

Reason for caution #2: Corporate earnings have been robust, which is Wall Street-speak for we somehow kept the lights on without installing a real business model.

Reason for caution #3: The big drivers are trading desks with risk-on energy, fueled by caffeine and the belief that tomorrow’s profits can be borrowed from the future.

Reason for caution #4: Interest rates are perennially in flux, meaning today’s party could be tomorrow’s hangover if the Fed pauses or hikes with a dramatic flair.

Reason for caution #5: Valuations are lofty, which is a fancy way of saying investors are betting that the next five years will resemble a montage and less like reality.

Reason for caution #6: The market’s optimism is highly concentrated in a few names, leaving other stocks to politely pretend they’re not being left behind.

Reason for caution #7: Inflation data bobs in and out of focus, like a remote control that keeps changing inputs just when you’re about to celebrate.

Reason for caution #8: Global politics continue to be a gravity well for capital, pulling dollars toward safe havens and risk assets in apparently equal measure.

Reason for caution #9: Many investors say they are long-term, even as they monitor every tick with the intensity of someone predicting the exact second their hoverboard will run out of charge.

Reason for caution #10: Finally, a stock market milestone is a reminder that numbers can go up while reality keeps a different schedule, so keep your diversification and your sense of humor.