Contradiction with Acts 10:44
Demonstrates that the Holy Ghost fell on Gentiles earlier, contradicting the view that Acts 11:15 was the first instance.
Acts 10:44: While Peter yet spake these words, the Holy Ghost fell on all them which heard the word.
Contradiction with John 20:22
Jesus giving the Holy Ghost earlier contradicts the event in Acts 11:15 being unique.
John 20:22: And when he had said this, he breathed on [them], and saith unto them, Receive ye the Holy Ghost:
Contradiction with Acts 1:5
Jesus foretold the baptism of the Holy Spirit, suggesting it wasn't unexpected as Acts 11:15 implies.
Acts 1:5: For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence.