I see the following around line 194:
When erf() returns a very small value, this cancels in the addition with the "1.0" which leads to poor precision in the results.
I would suggest changing this line to read more like:
Should you want some test cases for "extreme values" (one might argue that within 10 standard deviations isn't all that extreme) then you can check the following: http://www.jstatsoft.org/v52/i07/ then look in the v52i07-xls.zip at replication-01-distribution-standard-normal.xls
I think you will also find that evaluation of expressions such as
are pretty far off.