Library books, or other books with lettering on the page-ends – some people leave book-marks between the pages, and/or leave the books open before Shabbos. I don’t know what people do as far as closing those books or reopening them if they got closed on Shabbos.
Frosting on cakes – I remember learning that letters made with honey are OK, but otherwise there may be a problem.
Unfortunately, I don’t remember mekoros, and don’t have time to look things up now.
I seem to recall “k’sav ha’ussuy lemachak” (writing done with the intent of having it erased) is a kuleh.