Lesson Learned from Software Testing:
Many of us know, UX stands for User Experience and UI stands for User Interface.
Lets see an example:
In most of the websites we see a search engine.
Now, without entering any keywords in the search box and click on Search Button.
We don't see "No Error/Warning message". We feel that is a UI bug.
But, it has been worked as designed - to "re-enter and click on search button". Hence, the issue may be rejected as it is not UI Bug.
And a tester raises an Issue - There should be warning message "Enter a Search Keyword".
It is a UX Bug.