Contradiction with Matthew 23:8
This verse speaks against calling anyone teacher or master, while Hebrews 5:12 suggests a need for teaching.
Matthew 23:8: But be not ye called Rabbi: for one is your Master, [even] Christ; and all ye are brethren.
Contradiction with James 3:1
It warns against many becoming teachers due to stricter judgment, which contrasts with the encouragement in Hebrews 5:12 for believers to be teachers.
James 3:1: My brethren, be not many masters, knowing that we shall receive the greater condemnation. [condemnation: or, judgment]