From the original report:

If there are warnings and error displayed in a QML file and you add a "CMake-style" # (hash) comment by "accident" at the end of the file instead of using // or /* then the entire file's warnings and errors are removed.
Code completion within as-yet unclosed JavaScript blocks doesn't work at all. e.g.:

Button {
onClicked:

Unknown macro: { someObject.property = 10 // you will get no code completion within this block until the closing "}

" is added

} // Button

The new error-tolerant QML parser should support this case (and probably just ignore the line starting with '#').