Some lav doors are single panel and open outward (like normal doors) and some lav doors are bi-fold doors that you push inward to enter from the outside (and pull inward when you're inside and want out). I think I've been on planes that feature both types - so I can understand passenger confusion. Add in some alcohol, fatigue, mild oxygen deprivation and the inherent stress in travel and no wonder passengers don't seem to always get it.