Other comment mandatory
From LimeSurvey Manual
'Other:' comment mandatory (other_comment_mandatory)
Only applies to mandatory questions having an 'other answer' in which the participant can give a free text response ('other comment').
When this setting is activated and the participant selects the other answer (the 'other checkbox' or 'other answer in a list'), then he will have to give a comment in the 'Other comment' input box in order to proceed to next page.
Note that this option is not available for Multiple Choice questions because the Other checkbox status is not recorded in DB for this question type, only the content of the other text input is recorded. This means that checking the other checkbox is just a visual effect that is only meaningfull if the text box is filled in: that is why other_comment_mandatory is assumed to be set for this question type.
If activated for a "Multiple options with comments" question and the user enters a value in the left part of the Other answer but doesn't enter text in the Other Comment part, a warning message will be displayed when he tries to proceed to the next page.