The problem can occur if you open a file that had previously be saved as a Lotus 1-2-3 file, such as .WK4 format. It is also necessary that the file contains numbers that are greater than 16,385 -or- The file contains dates that are later than November 9, 1944. "This problem does not occur with numbers and dates that are returned by formulas." Microsoft is working on a fix.
Jeff Bagby, who first reported this to MacFixIt, notes more specifics of the symptoms: "Dollar values entered as $930.78 became $935.78. Dates of 3/17/1998 became 7/17/2076. We have seen this with numbers much lower than the 16,385 that Microsoft claims."