Q:

Asp classic formatnumber not showing final zeros

That function has always worked the right way for me, showing the decimals even if it's .00. Now it's not showing the final zeros.
Example:
<%= FormatNumber(1000,2) %>

Returns me 1000.
<%= FormatNumber(1000.99999,2) %>

Returns me 1000.99999.
Does anyone know what the problem can be?

A:

I'm going to go ahead and add this as an answer, since I just tried it out and got exactly the symptoms described.
It looks to me like there is a custom function somewhere in your code named FormatNumber. If a function is defined more than once, VBScript will use the most locally-defined version. In other words, if you write a function and give it the same name as a built-in function, your version will be used, not the built-in one.
For the built-in FormatNumber function, all of the arguments except the first one are optional, so FormatNumber(1000) will work fine, in the sense of not causing an error. If you're writing a function yourself, however, you can't do optional arguments, so if the function expects two arguments and you call it with only one, you'll get a syntax error. This is exactly the behavior you describe seeing.
Check through your page and all the include files (if any). I'll bet you dollars to donuts you'll find something like the following:
Function FormatNumber(inputNumber,ignoredDecimalDigitsParam)
'... possibly a bunch of code, all boiling down to:
    FormatNumber = inputNumber
End Function

