Product and version
FileMaker 14 and FileMaker 15 (probably earlier versions too)
OS and version
The Open Record script step fails if the cursor is in a calculation field or non-modifiable field (e.g. a field with "Prohibit modification of value during data entry" selected in the auto-enter options).
How to replicate
Place cursor in a calculation field or non-modifiable field and run the Open Record script step. The "This action cannot be performed because this field is modifiable" dialog pops up and the record does not open.
Check if the record is open already. If so, do not attempt to open it. If not, commit the record (this takes the cursor out of the field without triggering OnRecordCommit steps, though it does trigger OnObjectExit script steps on the field) and then run the Open Record step. It's not easy to get the cursor back into the original field though. You'd need to use object naming which is tiresome.