Contradiction with Matthew 14:21
It states that women and children were also fed, while Luke 9:15 implies the crowd only consisted of men.
Matthew 14:21: And they that had eaten were about five thousand men, beside women and children.
Contradiction with Mark 6:44
Similarly implies about five thousand men were fed, not including women and children.
Mark 6:44: And they that did eat of the loaves were about five thousand men.
Contradiction with John 6:10
Refers to there being about five thousand men, similar to what Luke 9:15 states, not indicating the inclusion of women and children.
John 6:10: And Jesus said, Make the men sit down. Now there was much grass in the place. So the men sat down, in number about five thousand.