I was creating a multi-page document with quite many formulas in it (Insert->Object->Formula). It happened sometimes, but I do not know any exact steps, that a formula content of an unrelated formula object was suddenly lost. The object was still there, but it was completely empty. You can imagine what surprise and disappointment such data lose can cause.
I tried to reproduce this on will, but I wasn't able to. It seemed to happen when writing near the end of the page, but also when editing old formulas. The last time it happened was when I was exporting the Writer document into a PDF and correcting some mistakes I would cause. I noticed after one such correction and export that one of the formulas has missing content, even I didn't touch that formula at all (at least not directly).
This is in fedora 21, libreoffice 4.3.4-1, x86_64.