Ok, you ate something you probably shouldn’t at this stage but you realise this & that’s the positive. And yes, some people do slip at any time as they’re progressing so you’re not alone.
The real question is what are you going to do next time? This is a great learning opportunity. Why couldn’t you resist the nachos? Was it something you wanted? Was it out of habit? Was it because others were eating? Did you need to eat? Was it your lunch or dinner time? Etc. Once you’ve worked out why you can start thinking about how you can manage this or similar situations in the future. Like, next time I go to the movies I’ll drink water or tea or I’ll sneak in a shake, some jerky or string cheese, etc.
Eating nachos now while you’re losing isn’t a great idea but in the future sure you may be able to have some but you’ll work that out.
PS - Actually you didn’t skip dinner. A shake is a meal replacement so you still had a meal. Just something to watch if you are on a three meal a day plan but plans are different.