- Open a new spreadsheet.
- Enter a value between -1 and 1 in any cell; 0 works too.  The bug will not
occur if the value rounds to -1 or 1, however (e.g. if you put in .9999999999)
I've confirmed this bug with .1, .01, 0, -.1, .437, -.9876, etc., and I believe
it will occur with all values that are between -1 and 1 after rounding to the
chosen number of decimal places.
- Right-click the cell, choose "Format Cells", set Leading Zeroes to at least 4
(it also works with 6 or 7 or 9, probably more), set Decimal Places to at least
1 (2 also works, probably more), and check "thousands separator".  This can also
occur with currency formats -- that's where I discovered the bug.
- Click OK.
- The last "thousands separator" will be after the hundreds place, not the
thousands, and any before that will be after the hundred-thousands,
hundred-millions, etc., instead of the millions, billions, etc.