Following are some key challenges of Software Testing:

1) Testing the complete application:
It is quite impossible to test the complete application. There are many test combinations so it’s impossible to test every combination. It will delay to ship the product, if you test all the combinations.

2) Relationship with developers:
It is a big challenge. In this case, skilled testers can easily manage this situation positively. Developers or testers can give many justifications when they don't agree with each other for some points. For this, they requires good communication, troubleshooting and analyzing skill.

3) Regression testing:
When your project expands, then the regression testing work turn out to be unmanageable. There may be so much stress to manage the changes in current functionality and previous working functionality checks and bug tracking.

4) Testing always under time constraint:
When manager gives you an order to ship the project under a specific time then testers makes their main aim to simply complete the task and they do not focus on the quality of the product. There may be many tasks in terms of quality which needs to be completed within specified time.

5) Which tests to execute first?
If testers are facing the problem related to time constraint, then they will also face the problem to set the prority of the test cases. For this problem, it requires good experience to work under pressure.

6) Understanding the requirements:
If testers are unable to understand the requirements properly then it will lead to bad testing because they will be unable to test the website or application properly. They requires good listening and understanding capabilities.

7) Decision to stop the testing:
It is a very difficult decision. It requires skilled testers who can go through the testing processes and importance of each process and then can decide to stop the testing.