Introduced in #2143 and pointed out in #2143 (comment):

Note: One side effect of the changes here (specifically 4645a4a) is that making changes to non-story posts when in Reader mode will start to show the validation error messages. Previously the validation warnings would only be shown when in Native/Transitional mode. The validation notices will likely be a source of complaints for users when in Reader mode, since they would be reported as if the Transitional mode is active.