Thursday’s wild ride is just a preview of coming weeks as investors try to digest the riots in Greece, mushrooming default fears in Europe, and sharp shifts in currency and bond rates – not to mention decidedly mixed signals from the U.S. economy.

Stocks plunged as much as 9% in heavy trading Thursday, while the dollar hit a 14-month high against the euro and the 10-year Treasury hit 3.41% — its lowest level since December.

“There is a lot of craziness going on,” said Andrew Barber, an investor who has been making money betting against stocks – and losing it betting against Treasurys.

Presumably, not every day will be as crazy as this one, which seems to been made much worse by a trading error in Procter & Gamble (PG) stock, as well as apparent missteps in trading of stocks such as Accenture (ACN). It is listed as trading as low as a penny Thursday afternoon — down from $41.

Stocks traded lower throughout the day before suddenly dropping like a stone in midafternoon, briefly dipping below 10,000 on the Dow Jones Industrial Average for the first time in three months.

P&G said the low trade in its shares was $56, down $6.12 from Wednesday’s close. But market quote services showed the stock traded as low as $39.37. “We’re looking into whether those were errors,” a P&G spokeswoman said.

The bizarre trading in P&G shares may have accelerated the marketwide decline by sending computer sell programs into effect. “We were in algorithmic trading land after we were down about 4%,” said Daniel Alpert, managing partner at New York investment bank Westwood Capital.

But the midafternoon plunge aside, few were shocked to see stocks tumble following a months-long rally in which volume was relatively weak. After rising steadily for more than a year, stocks are trading at high multiples to their earnings power. Accordingly, more and more trading in recent months has been speculative rather than value-oriented.

“There was not a huge amount of conviction on the way up,” said Alpert.

Barber, who runs investment manager Waverly Advisors in Corning, N.Y., has been expecting stocks to pull back for several months. He says his firm remains short on the S&P 500, though it has taken profits on that trade and is less certain that stocks will keep falling after their recent declines.

He sees the European authorities’ failure to take decisive action in the sovereign debt crisis sweeping Greece, Portugal and other Southern European nations as weighing on stocks and is likely to continue doing so.

“The Germans are between a rock and a hard place here,” Barber said. “Helping Greece sets a precedent, but there just isn’t the money to keep saving all these economies.”

As dire as the European situation appears to be, Alpert believes the bigger catalyst for this week’s selloff comes from the weakening U.S. economic picture. He said weak retail sales, soft income data and falling unit labor costs all point to the threat that the economic recovery will founder.

Alpert expects to see house prices start falling again. He says all the evidence of economic weakness here means the Fed likely won’t be able to raise rates this year, as the markets seem to be expecting.

“The market is starting to wake up to the fact that the economy may not have enough velocity to get out of the recession,” Alpert said. “It’s a little bit of a tinder box.”

And if the past few years have shown anything, it’s that once that tinder gets lit it’s awful hard to snuff it out again.