I don't know if this is a bug or not. It could be that I am just expecting too much of my PB360. Since I just got it I have been playing around with it, of course, and I've noticed that if you change scale (text size) or readers (mine has a choice of either Adobe or FBreader it tends to "crash" unexpectedly, that is, return to the start page ("Bookland"), and it's tedious to have to enter the book again and page through it to find the place where you stopped reading.
Everything seems to work fine if you don't change readers or scale, and it seems understandable that it will get confused otherwise, because how would it keep track of bookmarks and "last page opened" if you switch back and forth? Adobe has about half the total number of pages as FBreader--why I don't know and so far I see no particular advantage of FBreader.
Anyway, there seem to be some things that you shouldn't do if you want to keep your place in the books you are reading, and it would be nice if PB made these rules clear.