When the hooks are stored in the database, ALL the hooks using the process_templates_complete location are stored 'on top of each other' (one after another). If one is only 10 lines, then the next is 70 lines, then line "60" or the error could be line 60 of the second plugin if it was stored first OR line 50 of the second plugin if it was stored second. So, you actually need to look at all the plugins using that hook location to find the problem one.