Deming's principle no numeric target or goal should be set

This is not the best answer but something like this would help

1. If numeric goals like zero defects are set in IT organization , then it would de-emphasize the concept of optimum testing. Many times its not feasible to test everything and it may make business sense to release the software with list of bugs which will be fixed in the future releases.

 Setting goals on defects may also increase the cost of testing and programmers will focus on reducing number of defects rather than focusing on the severity or priority of them.

2. Motivation is also affected by imposing a numeric goals. Not all projects are equal and also its human to make errors. If we post numeric goals like 1% defects allowed or zero defects then the employees who constantly miss the target will be demotivated.

Deming believed that slogans like "Zero Defects" are actually counterproductive and may deemphasize the culture and tools associated with continuous improvement.

3. Not all defects are bad. Defects actually show the gaps in process. If we set numeric goals on defects in IT then there may be a tendency to hide some defects. Some defects may not be reported and it will be counterproductive for organization.  

 Also it will affect continuous improvement cycle.

You can elaborate on this with some more examples as per your discretion according to the marks.