I just got back from a trip Thursday night. I was away for a week, wasn't logging, am a stress eater and had the airports and airplanes and other crowds to deal with, was involved in a fender-bender, and I was with my grandchildren who are mostly fed healthfully but do enjoy a LOT of treats. So I basically binged for almost a week out of stress and freedom and all sorts of things.
When I got home, I didn't weigh myself - thought I would hydrate and go back to eating on plan and logging but let any retained fluid work its way out first. But I weighed this morning - and I was a "huge" 0.6 lbs higher than when I left!
So just as it's hard work to lose the weight in the first place, it's also real work to put it back on again. You have to "fall off the wagon" - eat relentlessly in your former pattern for days and weeks - in order to gain back any real weight. One weekend, one party, one trip where you see grandchildren you've been missing for 16 months will not do it.
Hope this helps allay some fears!