Side-effects and exceptions have to be completely unexpected to be real fun...

Developers of software for medical applications sometimes show a good sense of humor as the following lines in a highly mission critical Session-Manager show:

bool IsParticipantAttachable(Doctor doc)
{
  if (!Session.Attached(doc))
  {
    Session.Attach(doc); // attach the doctor to the session
    return true;
  }
  else
    throw new IllegalSessionStateException("doctor not attachable");
  return false;
}

... and now guess what happened to the piece of code that tried to ask the system twice, if a doctor could be attached to a session or not...