Contradiction with Genesis 13:15
God promises Abram all the land he sees, forever, which contrasts with the specific land allotment described in Joshua 18:20.
Genesis 13:15: For all the land which thou seest, to thee will I give it, and to thy seed for ever.
Contradiction with Numbers 34:12
The boundary description seems different from the one in Joshua 18:20.
Numbers 34:12: And the border shall go down to Jordan, and the goings out of it shall be at the salt sea: this shall be your land with the coasts thereof round about.
Contradiction with Deuteronomy 11:24
The extent of the land promised here stretches beyond what's described in Joshua 18:20.
Deuteronomy 11:24: Every place whereon the soles of your feet shall tread shall be yours: from the wilderness and Lebanon, from the river, the river Euphrates, even unto the uttermost sea shall your coast be.
Contradiction with Joshua 15:12
The western border here might differ from the boundary mentioned in Joshua 18:20.
Joshua 15:12: And the west border [was] to the great sea, and the coast [thereof]. This [is] the coast of the children of Judah round about according to their families.
Contradiction with Judges 1:18
Here, certain territories are mentioned as being captured, potentially conflicting with their allocation in Joshua 18:20.
Judges 1:18: Also Judah took Gaza with the coast thereof, and Askelon with the coast thereof, and Ekron with the coast thereof.