The rocky market action of the past week or so has prompted noted investor Dennis Gartman to issue a warning to his fellow investors: Be careful.
"The volatility of the past week and a half is unlike anything I've seen in my career, and I've been at this 40 years."
Despite rallying earlier in the day, U.S. stocks closed mostly lower Wednesday after the European Central Bank revoked a waiver that allowed Greek banks to use its government debt as collateral for loans.
Read More Greek news thwarts market rally
News like that can "come out of nowhere," Gartman said.
He believes the issues between the ECB and the Greek government will ultimately be resolved.
"These are people at the poker table and these are very high-stakes games. In the end, Greece is … going to be allowed to come out with some sort of bond issue. They're going to extend their debt, there's going to be some debt forgiveness."