This is an overview of all known errors which can occur while executing queries or while saving a widget. Errors can be annoying but this documentation might help you fixing your problem.
Syntax errors are mistakes in the syntax and should (or better not) occur when saving widgets or executing queries.
Runtime errors are problems which do not occur while saving a widget but will cause a widget to crash when executing.
Validation errors are problems which should be caught before a query is being executed. Those errors are being caught by an internal query validator to improve error feedback.