Contradiction with Mark 16:17
This verse states that believers will speak with new tongues as a sign, which contrasts with the warning tone in 1 Corinthians 14:21 regarding speaking in unfamiliar tongues as a lack of understanding.
Mark 16:17: And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;
Contradiction with Acts 2:4
This passage describes the apostles speaking in tongues as a Holy Spirit event, contrasting with the rebuked notion of unfamiliar tongues in 1 Corinthians 14:21 given in the context of prophecy and signs for unbelievers.
Acts 2:4: And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
Contradiction with 1 Corinthians 14:22
This verse says tongues are a sign for unbelievers, not believers, which contrasts with the purpose in 1 Corinthians 14:21 of tongues being a form of judgment or lack of comprehension for the Jews who do not listen.
1 Corinthians 14:22: Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying [serveth] not for them that believe not, but for them which believe.
Contradiction with Isaiah 28:11-12
This Old Testament reference, which 1 Corinthians 14:21 quotes, suggests that foreign tongues are a method God uses, yet here it implies a lack of acceptance rather than a spiritual gift as shown in other New Testament verses.
Isaiah 28:11-12: For with stammering lips and another tongue will he speak to this people. [stammering...: Heb. stammerings of lip] [will...: or, he hath spoken]
Paradox #1
The contradiction or inconsistency might be related to the source quoted. Paul references a prophecy in "the Law" which actually comes from Isaiah, a prophetic book rather than the Torah. This could be seen as inconsistent because "the Law" usually refers to the first five books of the Old Testament, not the prophetic writings.