Updated to 2.5.3 and now get these errors when submitting a safecracker form - it worked in the previous version.
A PHP Error was encountered
Severity: Notice
Message: Undefined index: checkbox_fields
Filename: libraries/safecracker_lib.php
Line Number: 1210
A PHP Error was encountered
Severity: Notice
Message: Undefined index: checkbox_fields
Filename: api/Api_sc_channel_entries.php
Line Number: 66