Source: Epub Secrets
Common typographic details that are easily solved in InDesign can be a big hassle in eBook production. One such detail is the handling of breaks between words and em dashes or ellipsis. InDesign (correctly) assumes these punctuation marks need to stay with the word previous to it, but ereaders will not always keep the word and punctuation together.
In most cases you will be lucky and won’t have bad breaks happen right between a word and an em dash, but if you work with a writer who uses them a lot or on narrow reading devices (like phones) you’ll probably see this [...]