Just before last Christmas, I bought a reduced advent calendar and shoved it in a cupboard for this year.
Opened the first three windows, and all were empty - I assumed gifts or whatever had all fallen to the bottom. On giving it a good shake, I realised no noise, or weight at the bottom. Opened it and realised that it's completely empty - obviously designed that you fill it yourself.
I'm not that annoyed - I only paid £1 for it, and I'm going to fill it with cat treats (she won't know she's missed a few days) - but if I'd purchased it for someone else would have been embarassing.