Summary updated until comment #101.
(The following happens in D7. The comment module in D8 has no problems, according to #99 and #101.)
After changing the comment_body field of a content type, the following warnings appear:
1. after deleting the comment_body field: when opening admin/content/comment I get this error:
Notice: Undefined property: stdClass::$comment_body in comment_admin_overview() (line 108 of /modules/comment/comment.admin.inc).
2. after deleting the comment_body field: when commenting on a node of that type:
Notice: Undefined property: stdClass::$comment_body in comment_submit() (line 2183 of /modules/comment/comment.module).
3. after making the comment_body field not-required: when commenting on a node with an empty body field:
Notice: Undefined offset: 0 in comment_submit() (line 2197 of /modules/comment/comment.module)
4. after setting the filter of the comment body to plain text, and clicking on the preview button to show the preview of a comment:
Notice: Undefined index: format in comment_preview() (line 2071 of /modules/comment/comment.module) (from #2077901)
The comments get submitted without problems but i don't know what's the error about.