Hey, it happens in the 12th position with “Precise Math” turned off, too.
I just turned it off, then made a version of the test code that only uses 12 places after the decimal, and the problem still happens:
<cfset my_number = 0.111111111111>
<cfset my_formatted_number = NumberFormat(my_number,"0.000000000000")>
<cfdump eval="my_number">
<cfdump eval="my_formatted_number"><hr>
<cfset my_number = 0.999999999999>
<cfset my_formatted_number = NumberFormat(my_number,"0.000000000000")>
<cfdump eval="my_number">
<cfdump eval="my_formatted_number"><hr>
<cfset my_number = 0.415661687696>
<cfset my_formatted_number = NumberFormat(my_number,"0.000000000000")>
<cfdump eval="my_number">
<cfdump eval="my_formatted_number">The formatted numbers output are these (run it yourself to verify):
0.111111111112
1.000000000000
0.415661687697
So it even happens when using doubles, not just BigDecimals. Pretty serious bug that can cause hard-to-notice numeric inaccuracies. FYI.