+@item
+@code{isnan} was introduced with C99 and is thus commonly not present
+on pre-C99 systems.
+@item
+On IRIX 6.5 with @code{cc}, @code{isnan} does not recognize some NaNs.
+@item
+On NetBSD/i386 and glibc/ia64, @code{isnan} does not recognize some
+forms of NaNs, such as pseudo-NaNs, pseudo-Infinities, and
+unnormalized numbers.
+@item
+On i686 and @var{x}86-64, @code{__builtin_isnanl} (and thus
+@code{isnan} implementations based on it) in GCC 4.0 and later does
+not recognize pseudo-denormals as NaNs, and similarly for
+pseudo-zeroes, unnormalized numbers, and pseudo-denormals on ia64.