If you are trying to select a JSON field with a number or dash, you may get a jq compile error. As mentioned in a GitHub issue, the key gets parsed as a substraction so we need to enclose it in additional quotes.
WordPress 3.2 upgrade, WYSIWYG editor no longer loads? Now requires JSON extension
An issue I encountered recently doing a WordPress upgrade from 3.1 to 3.2, is that afterwards everything appeared to work with the exception of the editor for Posts and Pages. After working through the checklist for the manual install and checking each plugin individually, I took a look at the PHP error log and noticed that PHP didn't have the JSON extension bundled.