The worst was probably when I set a record (I believe) for the fewest books sold at a book reading. It was in Singapore and only a handful of people showed up and nobody bought a book.
The only person interested won the book I was giving away as a prize! So I ended up with sales of negative one books - a record perhaps?
Another time I was talking to a big group in a big book shop and somebody walked in front of me and asked me where the toilets were!
My best experience was addressing many hundreds of kids at a brilliant book festival in New Delhi called Bookeroo. There were authors and children everywhere united by their love of books.