I think it depends on where your head's at... if you're talking about breaking the plan, and you're the sort of person that can do that and get back on the horse, then I guess the only downsides might be that a) you might come out of ketosis, b) it makes it more of a risk that you'll do it again.. and then maybe again.. etc on future days... if you stick with your current plan, you can feel so very proud of yourself... chances are, you're thinking this because you're either feeling: tempted, hungry, angry, bored, lonely or tired.. they're my main reasons, and usually it's better to do something that actually addresses these feelings, rather than mask it with eating..
..I guess we're all different.. it may simply be a 'one off' thing, and then end of story - I guess that's for you to have a conversation with yourself about, cause ultimately, this is entirely about what YOU want for YOU, and noone else...
All the best, and keep us posted
Vx