Challenges faced while working on Selenium

There are many challenges we face while working on selenium. Some of the challenges are:

1. configuration challenges on different machines
2. selenium's compatibility with browser versions like with FF20 is not supported with some web-driver versions.
3. handling IE/chrome etc.
4. popup handling
5. Ajax handling
6. security certificate handling
7. iframes
8. date pick from calender.
9. identifying locators
10. Upload and download file
11. Maintain the scripts to work properly irrespective of new build 
12. Diff to identify script has failed may be unable to locate the element or config issue(Browser updated) if your framework is not able to catch issue.
13. If you are using xpath as a locator. writing xpath without using dynamic changing data. i.e., xpath generated dynamically.