What the hell?

From Microsoft's Knowledge Base:

When you enter a negative number in the ERF function or in the ERFC function, you receive a "#NUM!" error in the Microsoft Excel worksheet. The "#NUM!" error occurs when you enter a negative number as the lower_limit value or as the upper_limit value.

Additionally, if you enter a numeric value of 27.3345 or larger in the lower_limit value or the upper_limit value in the ERF or ERFC functions, you may receive a #NUM! error in the Excel worksheet.

CAUSE:
This problem occurs because Microsoft Office Excel 2003 and earlier versions are not designed to correctly process negative input values and input values that are larger than 27.3345. This is true even though negative input values and input values that are larger than 27.3345 are both mathematically correct.

Gee, thanks. What if I actually need to, say, compute erf(-1) for, say, comparing against a normal (bell curve) distribution?

Goddammit. Microsoft, you suck. I shouldn't have to write my own program to do something that should be so freaking trivial in Excel.