In an effort to combat the principle of Sola Scriptura that is both explicitly (Deuteronomy 4:2; Proverbs 30:5-6; Amos 3:7; 1 Corinthians 4:6; 2 Timothy 3:16-17; 2 Peter 1:20-21; Revelation 22:18) and implicitly (e.g. Acts 17:11) taught throughout scripture, many Catholics argue that the Bible itself claims that Jesus had many things to teach that are not taught in scripture. There are two main verses they cite in defence of this: John 21:25 and John 16:21.
In John 21:25, we read "And there are also many other things that Jesus did, which if they were written one by one, I suppose that even the world itself could not contain the books that would be written. Amen." Now, there are two important things to note here. First, it doesn't literally mean that all the works of Jesus could not be written in the Bible. The world itself is certainly large enough to contain every detail of Jesus 33 year long life, so it's clear that John is using the common literary technique of exaggeration to emphasise a point. The other 3 Gospels mention things that John's Gospel does not, so it's clear that they simply picked what they, being lead by the Holy Spirit, thought was important to mention.
Second, specific miracles done by Jesus don't necessarily add new doctrines. Miracles were used as a sign of authenticity as Jesus both performed that which God alone can do and fulfilled ancient prophecies about what the Messiah would do, verifying His identity. We didn't get a new doctrine every time Jesus performed a new miracle, and so even if we find extra traditions about miracles He may have done that are not recorded in scripture (these traditions do exist), they will at best be a piece of historical interest.
A third important thing to remember at this point is that Catholics severely misunderstand the Christian view on tradition. No Christian I know believes that all tradition is bad, just that tradition is fallible, and must be second to the Bible. If tradition is not in the Bible, but does not conflict with it, it's not a bad thing, and a Christian is free to observe or abstain (Colossians 2:8, 16-17). If a tradition is in the Bible, such as Baptism, it must be observed (though not to the extent where a lack of obedience results in Hell, or any time in the mythical realm of Purgatory). If a tradition is antithetical to the Bible, such as a wide range of Catholic doctrines, it must be scrapped. This ultimately means that while scripture is the sole and sufficient authority of Christianity, it is not the only authority Christians are permitted to believe. After all, every church service from every denomination would be an abomination without such a principle. Point being, no one has any problem with the possibility that some of the things Jesus did were not recorded in the scriptures.
But the really naughty trick is when Catholics claim that Jesus had many things to teach that were not recorded in scripture. Jesus never said any such thing! What He did say is "I still have many things to say to you, but you cannot bear them now". Now, does that line up with the Catholic's claim that Jesus said His teachings aren't all in scripture? No! He says "...but you cannot bear them now". Jesus wasn't saying to the whole Church "you need more than my words, go to this blatantly unBiblical Church that thinks it's morally acceptable to communicate with my mother, whom they erroneously claim was sinless, and they'll tell you the rest". He was saying that at that point in time, the Apostles were not ready to receive the teachings that we find in the rest of the New Testament. This isn't surprising, given that He hadn't even been arrested yet.
Further proof that Jesus was not referring to the traditions of the Catholic Church comes from the fact that the surrounding context tells us what He was referring to. Jesus was telling the Apostles that when He left, He was going to send the Spirit of Truth, who would guide them, the Apostles, into all truth.
After His death and resurrection, Jesus did, indeed, teach the disciples more things, after which He did leave, and He did send them the Holy Spirit, who did lead them into all truth, guiding them to write the scriptures so that we, too, may know the truth. That same Holy Spirit uses you as His Temple (1 Corinthians 6:19), and is essential to both your faith and your understanding of the scriptures (1 Corinthians 2:6-16). Most importantly, He teaches you what they say! (1 John 2:27). This, itself, is contrary to the Catholic teaching that only the Church has the authority to interpret the Bible. If the Holy Spirit resides in each believer, teaching them the truth as they read the scriptures, then no Church is needed. The language barrier is the only thing that has ever stopped people understanding the Bible, but for speakers of every major language in the world, that barrier has been removed by brave men and women who have spent plenty of time, energy, money, and in the earliest days of the Reformation even gave their very lives translating it.
And so we see that this particular attempt to refute Sola Scriptura doesn't hold water. It goes far beyond the scope of the verses, twisting them beyond any recognition. "You cannot bear them now" does not mean "the Catholic Church will tell you later". Rather, prior to His resurrection, the Apostles would not have been able to understand a large portion of what Christ would eventually teach them through the Holy Spirit after He ascended to Heaven. Even within the very same passage, we read as much. Catholic apologists should be embarrassed that they think this is an argument worth putting out in public, and any Catholic who falls for it should be likewise embarrassed.