Various syntax errors in .json, .xml, .po and other resource files for your application or website which can result the failure of processing the files by the engine used by Text United. Therefor your file will not be processed for translation.
To check if your file is correctly structured and free of errors you can use one of these free validator tools that will display and point out possible errors in your file:
JSONLint - The JSON Validator:
YAML Lint - Validator for .yml files
PHP Code checker
XLIFF 2 Validation
Microsoft Visual Studio
(Visual Studio offers resource designers that validate the files automatically)