Most Protestants, especially those who would identify as Bible Christians or go by Sola Scriptura, like to say that they follow the authority of God's word in Scripture and anything else is unreliable and lacks any authority. In particular, they would criticize "unbiblical traditions" that are followed by others, including Catholics, and that do not have explicit support in Scripture, according to their view.
So it is somewhat ironic that these very same people follow several traditions not found explicitly in Scripture...and hold them as BINDING doctrine, not merely interpretations or optional customs.
Here are five of them:
1 & 2: Christmas and Easter. These feast days have been held by tradition for centuries and are certainly not commanded by the Bible! Here is a further explanation-
In order to be consistent, a Bible Christian would have to admit that no Christian is obligated to celebrate these two most holy days. This would basically strip out any remaining religious holidays for Protestants. They already threw out most of the Church calendar, but kept these two. Who made that choice? The other feast days are depicted in Scripture also, such as the Feast of the Annunciation (Luke 1).
3. Sunday celebration. Where does the Bible say that Christians need to meet together on Sundays in particular? Why not Fridays? This is a very well established tradition, obviously, but it is not explicitly commanded in Scripture. Here is a further explanation-
In order to be consistent, a Bible Christian would have to admit that no Christian is obligated to meet together on Sundays, as long as they do meet on some other day of the week, which would satisfy Hebrews 10:25.
So far, we are looking at Protestants who meet on a random day of the week, maybe a Thursday this week and a Monday next week, and have no Christmas and no Easter. Does that sound very Christian to you? That is to be consistent with only what is explicitly required for Christian practice in Scripture. But wait, there's more.
4. The Doctrine of the Holy Trinity. Whoa whoa, the Trinity!? Yes, it can surely be supported by Scripture but based on Scripture alone, it is possible to string together verses that do not support the Trinity. This was a major controversy in the early 300s when the priest Arius began teaching against the Trinity, which had been held by Tradition. Jehovah's Witnesses and Mormons have our same New Testament and deny the Trinity. They know how to read and they share these holy books but have a different interpretation. So how can a Protestant go to them and say that they have no right to interpret the Scriptures for themselves? On what authority?
In order to be consistent, the Protestant is reduced to conceding that, ultimately, if a person is well meaning and believes they are following the Lord, each has an equal validity to interpret Scripture for themselves, even if one concludes that the Father is greater than the Son and not equal to Him, for instance, which violates the doctrine of the Trinity.
Is this private interpretation like the early Church or not? See here-
5. The Canon of Scripture itself. Now, talk about circular logic. The Bible Christian says that only the Bible is authoritative...but has no infallible authority on which to rely that defines what the Bible is! The Bible was not written as one book with a table of contents. It is a collection of writings, so who decides (infallibly) what is included and what is not included which will define the Bible itself!? Houston, we have a problem!
See here for more on #4 and 5-
In the end, Bible Christians are just fine with certain traditions not found explicitly in Scripture. These are five doggedly held beliefs by Protestants that rely on extra-Biblical authority. In other words, these are "unbiblical" traditions of Protestants who claim to believe in Sola Scriptura.
In order to be consistent, the Protestant must admit that none of these are absolutely required by the Scripture (whatever the canon is anyway). Stripping away these, you are left with "Christians" who meet once a week sometime, do not celebrate feast days like Christmas and Easter, cannot require adherence to the Trinity, and also lack any way of even defining the Bible without using the authority of Tradition, which they deny. This sounds more like a weekly book club than the powerful Church of the apostles founded by our Lord Jesus Christ.