How, in the name of flip, did they not realise these were two different shoes when they packed them?

Melon-headed scribbler of words. Procrastinator. Hairy faced daydreamer. Writes children’s stories as a hobby, loves reading (Douglas Adams is my literary hero). Big fan of Fortean shenanigans. 📝🐕🏃🏻🌱💻
How, in the name of flip, did they not realise these were two different shoes when they packed them?