File numberformat25.xsl contains the following lines:
<xsl:decimal-format decimal-separator="!" grouping-separator="!" />
<out><xsl:value-of select="format-number(931.4857,'###!###!###')"/></out>
The result of transformation is: <?...?><out>931</out>
Since a parser implementation can consider that first ! in pattern is either decimal-separator or grouping-separator the result will be either 931 or 931.485.
Thus the parser bahaviour is implementation-biased.

The xsl:decimal-format element has a lot of attributes that control the format of result:
<xsl:decimal-format
  name = qname
  decimal-separator = char
  grouping-separator = char
  infinity = string
  minus-sign = char
  NaN = string
  percent = char
  per-mille = char
  zero-digit = char
  digit = char
  pattern-separator = char />
Since the order of handling isn't specified the result of transformation is implementation-biased if a parser does not control that all attributes has distinct characters.
It makes sense to add a control and throw an exception like this: The same character is used as the grouping-separator and as the decimal-separator