I have seen recently the following in writing:
This is a very simple, but yet an important observation.
I am almost certain that the [modern] use of but yet here is a typo, and the intentions were to use either of the two words (but the author might have hesitated which form to use to the point where s/he simply forgot to remove one of those).
The simple question is whether or not the sentence above is correct (and why?); but more importantly, I think it might worth sharing the story behind the archaic use of this strange expression. That is why I tagged this question accordingly.