I think that the biggest mistake that can be made is to think of Taijiquan as some kind of structural mechanics. If this were the case it ought to be fairly easy to master.
Instead it is about energetics.
Suspending the Headtop is a good example of this. For years I either imagined that I had a book on top of my head or a ponytail tied up to a rafter or beam in the ceiling. The trouble for me is that it resulted in a lot of tension.
I believe that misconceptions of this concept arise due to the difficulties in translating writings that were coded to support a oral/experiental process.
Louis Swaim discusses just this in his translation " Fu Zhongwen - Mastering Yang Style Taijiquan, (North Atlantic Press 1999) about the meaning of "an intangible and lively energy lifts the crown of the head" - p182.
Swaim quotes fromYang Jwing Ming, T.T.Liang, Robert W Smith, Jou Tsung Hwa and others,all offering diverse interpretations pp182-182.
What I find interesting is that we are talking about the wrong focal point, which is not the crown of the head. The crown of the head is commonly referred to as the Ba Hui point. Instead it is the Niwan point,which is not the crown but is located instead deep inside the brain. Scott Meredith discusses this extensively in his book "Juice-Radical Taiji Energetics" pp 76 - 83.
Sam Tam also echoes this point as the place from which to suspend the headtop in Jan Diepersloot's book " The Tao of Yiquan 1999, p215.
Scott Meredith likens the Niwan point to the Ajna in the Yogic tradition and one of my teachers, John Kells also worked with a point deep in the brain which he called the Ankh.
So whatever your take on this,there is plenty to work on. I'm off to practise!