In rosaries, an area I'm coming to know fairly well, such mistakes are sometimes quite amusing. I've mentioned the "1830 rosary" trap here before. Rather more challenging is looking at some of the pieces people put up for sale and trying to figure out if they are actually rosaries at all. Sometimes people will put together something out of various leftover bits (see Orts) and may or may not succeed in getting them in the right order to make a rosary.
Here's an example that turned up on the German eBay recently. As far as I can tell, this is silver filigree round beads from a Biedermeyer rosary, a cross from somewhere else, and some lengths of chain. There are no "Hail, Mary" beads at all, though admittedly the result is a rather pretty necklace if it's long enough.
There's also a certain amount of confusion that I think we can blame on Madonna -- the singer -- since she brought the idea of wearing a rosary as a necklace back into fashion (more on this another time). Now we have sellers offering "rosary necklaces" that are plain old ordinary rosaries, and we also have sellers putting together necklaces that are clearly not really rosaries but have the "look" and referring to them as rosaries. Count the beads before you buy