This is especially in reference to moon sighting where a witness report can possibly be in conflict with sound scientific principles. Below is the visibility map of the moon on July 19, 2012 (for determining the first day of Ramadan). As you can see, it is not possible to see the moon in the USA on that day. If an otherwise reliable witness from the USA comes forward and claims to have seen the moon, is he still considered an acceptable witness (even though, scientifically, his claim is considered impossible)?
Green - Easily visible with naked eye
Blue - Visible if perfect conditions
Gray - Optical aid to find moon
Red - Visible with optical aid only
Black - The moon cannot be seen by any means