I don't see how the scripture can say anything about marian apparitions (except, theoretically speaking, the one to St. James). However, if, hypothetically, I find a disagreement between parts of what the Church teaches, for example, a contradiction between the scripture and some other part of the General Revelation, then I will conclude that I do not understand the teaching in question. If however, I had stubbornly refused to acknowldege my limitation and insist on my own interpretation of the revelation, then I indeed would have committed a grave sin which absent absolution would lead to my eternal damnation.
Perhaps I should identify the sin by name, it is the cardinal sin of pride and a violation of the First Commandment.
The superstition of Sola Scriptura leads to that sin often.