Sloppy

Receiving correction is vital in any martial art but especially so in taijiquan.
 Careless, sloppy practice places stress on the joints.
 Your body quite literally cannot reasonably perform the demands being placed upon it.
 Also, it is not enough to simply copy/mimic - you must have some understanding of why you are doing things in a particular manner.
 This is why the lower grades are all about physical fitness and health.