I created a saved Slideshow in Lightroom 5 (version 5.6 for MacOS) with 267 slides. I used Text Overlays to display the Title metadata under each photo. It works fine for the first 208 slides, but on slide 209 it shows both the caption for slide 208 and the caption for 209 - overlaid. Wierd! This behavior persists, showing the current and previous slide's caption on each slide for the next ten or twenty slides. Then it resumes normal behavior for the final batch of slides.
I've looked at all the Slideshow settings along the right (Options, Layout, Overlays, Backdrop, Titles, Playback) and everything is the same for all the slides in the area of slide 209. Anyway, I can't see how this would be a desirable option one might turn on, so perhaps it's a LR5 bug?