When a non-existant bitmap file is given to a widget with X11 or Aqua Carbon Tk 8.4 on OS X, an exception is returned.  But with Cocoa Tk 8.5 or 8.6 on OS X, the error is not detected by Tk but does result in an OS X error message.

With X11 Tk (MacPorts 8.6.1)

<verbatim>
$
% if [catch {button .b -bitmap "blah"} result] {
    puts "Failed: $result"
} else {
    puts "passed"
}
Failed: bitmap "blah" not defined)
</verbatim>

With Cocoa Tk 8.6.1 or 8.5.15:
<verbatim>
$
% if [catch {button .b -bitmap "blah"} result] {
    puts "Failed: $result"
} else {
    puts "passed"
}
2014-01-31 14:58:07.134 Wish[9555:507] CFURLCopyResourcePropertyForKey failed because it was passed this URL which has no scheme: blah
passed
</verbatim>