Here is a screenshot referent to the 1917 Code of Canon Law about the Teaching Authority of the Church. The relevant part is 1166. In particular, this statement:
No part of the religious teaching is to be understood as dogmatically declared and defined, unless such a is clearly known to have been made (Canon 1323)
A teaching is not infallible unless it meets the conditions for infallibility. As simple as that. The confusion resides in the assumption that a fallible teaching, is necessarily an error, which is not so either.
Cantarella,
You don't seem to understand the subject we are discussing here. The above quote is referring to a solemn declaration from the solemn magisterium and is perfectly legitimate. That particular statement is not referring to the ordinary magisterium.
Maybe a real world example will help clarify how this all works. For the first 3 centuries after Christ, the entire Catholic world learned their faith through the ordinary magisterium (the continuous, unanimous teaching among the Pope, all the bishops, and anyone helping them). There had been no solemn teaching (solemn magisterium) yet. During those 3 centuries, Catholics learned about the divinity of Christ, which was taught unanimously across all bishops. All the bishops knew well about the promise that the Church could not possibly teach error, so they knew the doctrine on the divinity of Christ was true without a doubt, having been taught unanimously between all of them without the Holy Ghost preventing it. This is the infallible ordinary magisterium in progress.
When Arius started teaching contrary to the other bishops on the divinity of Christ, the other bishops tried to correct him, knowing with absolute certainty he was wrong based on what I just said above, even though there had been no solemn teaching yet. When Arius refused to correct his belief, the bishops circulated a letter saying he was guilty of teaching heretically against the continuous teaching of the Church (the ordinary magisterium). When Arius still refused to teach in unison with the other bishops, they called the Council of Nicaea and condemned him as a heretic, and at the same time solemnly defined the doctrine on the divinity of Christ so no one would make the same mistake in the future. This was the first time the Church had used the solemn magisterium - a full 3 centuries after Christ). After this, the infallible ordinary magisterium continued on its way. At this point, the doctrine on the divinity of Christ was now basically the only teaching of both the ordinary magisterium and the solemn magisterium.
Since then, the solemn magisterium has only been called to form a General Council 20 times in 2000 years, and each time it was simply to clarify a teaching of the ordinary magisterium that was being attacked at the time. Catholics historically have always learned their faith through the ordinary magisterium, which is guaranteed infallible. The infallible solemn magisterium steps in only in rare cases, like a referee would, to clarify and straighten out problems. Once the referee has done his job, the ordinary magisterium resumes day-to-day infallible teaching. Hopefully that helps clarify how the ordinary and the solemn magisterium work together to form one, continuous infallible teaching of the Church.