Contradiction with Deuteronomy 4:2
This verse instructs not to add to or take away from God's commands, while John 20:30 implies more signs not written in the Bible, suggesting incompleteness.
Deuteronomy 4:2: Ye shall not add unto the word which I command you, neither shall ye diminish [ought] from it, that ye may keep the commandments of the LORD your God which I command you.
Contradiction with Revelation 22:18-19
Warns against adding to the prophecy, hinting at the sufficiency of scripture, whereas John 20:30 suggests there are additional unwritten works of Jesus.
Revelation 22:18-19: For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: