In Deuteronomy we read that Moses was not to disturb the children of Ammon nor attempt to take their land:

"That the LORD spake unto me, saying, Thou art to pass over through Ar, the coast of Moab, this day: And when thou comest nigh over against the children of Ammon, distress them not, nor meddle with them: for I will not give thee of the land of the children of Ammon any possession; because I have given it unto the children of Lot for a possession."

Deuteronomy 2:17-19
 

"Only unto the land of the children of Ammon thou camest not, nor unto any place of the river Jabbok, nor unto the cities in the mountains, nor unto whatsoever the LORD our God forbad us."

Deuteronomy 2:37
 

Yet Joshua tells of how Moses gave the land of Ammon to Gad as an inheritance:
 

"And moses gave inheritance unto the tribe of Gad, even unto the children of Gad according to their families. And their coast was Jazer, and all the cities of Gilead, and half the land of the children of Ammon, unto Aroer that is before Rabbah;"

Joshua 13:24-25
 

God himself vowed to not give Moses (pbuh) even the smallest portion of the land of Ammon, yet Moses is now claimed to have taken it anyway (against God's will?) and given half of it to GC.E. Was this a slip of a scribe's pen? Is it a difference in "spelling"? Centuries of tampering with the word of God has left it's mark.