CSS Errors 

CSS errors (usually LESS processing errors) will be printed at the bottom of the compiled site.css file. Access it by browsing to http://yoursitename.squarespace.com/site.css and scroll to see error messages. If your site suddenly appears to be rendering with no CSS styles, it is a good time to check your site.css.

LESS Errors are returned in the compiled  /site.css file

Template Structure Errors 

Template file errors (mostly errors in misformatted template configuration files) can be viewed in the Developer tab.

Template Structure Errors are returned in the Developer tab.

JSONT Errors

Squarespace also reports JSONT related errors on your front end site directly.

JSONT Errors are hard to miss... they return a black screen of death on the pages they affect.