Bug
also: bugs, buggy, defect, defects
A mistake in how your app was built that makes it do the wrong thing, or fail to do the right thing.
A defect in code, configuration, or logic that makes software behave differently from what was intended. The bug is the underlying mistake; crashes, error messages, and wrong output are the symptoms you notice.
A recipe that says a tablespoon of salt where it meant a teaspoon. The cook follows it perfectly, and the soup is still wrong. The mistake is in the instructions.
Much of your work with an agent is describing bugs. The clearer you are about what should happen versus what actually happens, the faster it finds the real mistake.
Found the bug: the coupon is calculated from the original price instead of the sale price, so sale items get discounted twice.
There's a bug on the booking page: picking March 3 saves March 2. Expected: the date I pick. Find the cause before changing anything.
Thinking a bug always shows an error message. Many bugs are silent: the app runs fine but gives the wrong answer, like a wrong total or an email that never sends.