See tst_QScriptContext::backtrace() test, data tags "eval" and "eval in member".
If a backtrace contains a call to eval(), then the backtrace item corresponding to the function that calls eval() doesn't have the correct line number (it reports -1).
The comment in the test claims that this is only an issue when the JIT is disabled, but that's not true; with the JIT we hit an assert when NDEBUG is not defined.