It is impossible to create error/bug free product from developer or an expert people in certain technology. There are some reason that cause major bugs in product.

1. Writing Initial Requirement Miscommunication is the major issue while writing a requirement initially. Some more major issue of appearing bugs are people dont go through requirement properly and mutual misunderstandings made by a software team during specification, design, coding, data entry and documentation.

2. Errors in Code: As we know no one is perfect so human errors made while writing code and in these are normally syntax error, wrong positioning of loop and conditions. Errors can also occur when developer have to write 1000 lines in a single sitting. Some bugs appear if logic design is not proper.

3. Daily changes in requirement: Even a small and single changes in requirement daily, makes product difficult to understand the real work and work flow, this lead not only to affect on code but also have impact on integration of module and it also increase the risk of failure of product.

4. Target time to complete module:: Mistakes increase because developer cares more about completing their task on time rather than focusing on requirement and due to this errors in code occur and they lose some important work flow during development.

5.Lack of experience: Lack of experience in certain work or domain can lead to mistakes in product.

Hence, to keep bugs away, everyone who are involved in development of product should keep eye on Requirement, developer must get proper amount of time to complete the module and should feel free to discuss if he/she is not be able to understand the requirement properly. Developers should choose some better programming style, techniques and code analysis.