Ok, this is a weird one. I have a couple open translations in various stages of completion. Today I realised, that something strange happened to some of them. For instance I'm translating WP:MoS from en.wiki to my user space @ pl.wiki (namely https://pl.wikipedia.org/wiki/Wikipedysta:Halibutt/MoS, but I didn't save yet). I stopped looking at it for a couple of weeks and returned to the job only today.
I had the lede (the first five paragraphs) ready, with the text in Polish nicely shown on the right and the original text on the left. However, now the first paragraph of my translation is shown next to where the "Article titles, headings, and sections" heading is in the original text instead. Also, I translated the entire sub-section on "Article titles", yet it got moved to the very end, way below the end of the original document.
Shorter translation projects or those started only yesterday seem to work fine.
Windows 7x64, Firefox (42.0).