People can practice serve for several reasons -
1) to time and develop muscle memory for a sound technique
2) to make small changes to a sound technique
3) to make significant changes or to develop a new technique
For me, I tend to leave out several important things that I know are used in strong serves. For example, I often forget to get enough leg thrust, forget to have the handle of the racket point up toward the ball at the proper time, forget to have the racket edge toward the ball when starting the last acceleration to impact, and others. I need high speed video to observe these things. I have to get these things into my motion before practicing for timing.
In addition, the odd quirk will get into our serves -
See reply #184
I believe that this wrist position caused me some minor pain last summer when I started to practice serving. I cut the practice down to nil because of worrying about the wrist stress & pain.
See reply #14-
See replies related to bend in elbow at impact, replies located before and after reply #55 with a picture.
There are also the important safety issues related to technique such as the shoulder high orientation for the serve to minimize impingement risk. Just one very bad motion can cause injury.
1) Jim McLennan short video on the rotator cuff, impingement and serving
2) Todd Ellenbecker video on shoulder anatomy, impingement, and serving. At about minute 8 he describes the same issue as McLennan but in more detail.
If you are concerned because you are having pain, how can you determine that the technique that you use is OK? You have to study and know the proper technique and verify that you are doing it with high speed video or find a well qualified instructor. Keep in mind that the more rapid motions during the serve cannot be seen by eye or even 60 fps video so an instructor who uses HSV is a plus.