Under what conditions might instancesRespondToSelector: return true, but performSelector: throw an exception

I have code distributed in a library which looks like this:

if ([[NSString class] instancesRespondToSelector: @selector(JSONValue)]) {
  NSString *jsonString = [[[NSString alloc] initWithData: jsonData encoding: NSUTF8StringEncoding] autorelease];
  dict = [jsonString performSelector: @selector(JSONValue)];
}

For some reason a -[__NSCFString JSONValue]: unrecognized selector sent to instance exception is getting thrown when the performSelector: method gets called. This is code that is distributed in a library that I wrote, but I can't reproduce or debug it myself. Instead a third-party is reporting this problem. Under what conditions could instancesRespondToSelector: while actually calling the method using performSelector: throw an exception?

edit There is a case which could explain why this occurs, but it doesn't make sense. If the developers were to do something like this:

@implementation NSString (OurHappyCategory)

+ (BOOL)instancesRespondToSelector:(SEL)aSelector
{
  return YES;
}

@end

It would explain why the code is executing, but it would of course be a very bad thing to do. Is there a way this problem could occur that makes sense?