C-u 1 inserts four "1" characters?

[Sean McAfee]

I almost always specify command prefix arguments by holding down the
meta key while I type the argument digits.  But just now I tried
specifying the prefix by typing C-u followed by the digits, and as soon
as I typed the first digit, four copies of the digit were inserted into
my current buffer!

Further experimentation showed that it didn't matter how many times I
type C-u prior to typing the first digit; four copies are always
inserted (not 16, 64, etc).  Also, if I type a hyphen following the C-u,
I get four hyphens in my buffer.

Anyone have any idea why this could be happening?