Contradiction with Exodus 12:35-36
Exodus 12:35-36 describes the Israelites obtaining materials from the Egyptians, which contradicts the impression in Exodus 38:29 that all the bronze came as a freewill offering.
Exodus 12:35-36: And the children of Israel did according to the word of Moses; and they borrowed of the Egyptians jewels of silver, and jewels of gold, and raiment:
Contradiction with Exodus 25:3
In Exodus 25:3, contributions are made for the tabernacle, including bronze, indicating they already possessed materials, which contrasts with the implication in Exodus 38:29 that the bronze was voluntarily offered anew at that time.
Exodus 25:3: And this [is] the offering which ye shall take of them; gold, and silver, and brass, [offering: or, heave offering]