For compiled languages syntax errors occur strictly at compile-time. A program will not compile until all syntax errors are corrected. For interpreted languages, however, not all syntax errors can be reliably detected until run-time, and it is not necessarily simple to differentiate a syntax error from a semantic error; many don't try at all.
A syntax error may also occur when an invalid equation is entered into a calculator. This can be caused, for instance, by opening brackets without closing them, or less commonly, entering several decimal points in one number.