Text editor has some major problems with editing cyrillic text. Editing a large file with such characters leads to illogical cursor movements, garbling and therefore to the data loss.
It's hard to catch a minimal working bug example. Easiest way is to open attached file. It contains 29 bytes of UTF-8 cyrillic text and can be generated in UTF-8 enabled shell by
echo "вероятностей 123" > foo.tex
Then open a file in the TeXstudio, place a cursor just before the space and after letter "й" and press backspace. Text should be "вероятносте 123" but it transforms to "вероятностй 123".
I attached two screenshots, before and after backspacing. Bug appears only in a previously saved file, can't reproduce it in a freshly created editor tab.
mac os x 10.8.2, english locale
TeXstudio 2.5.2 (SVN 3651M)
Using Qt Version 4.8.4, compiled with Qt 4.8.4 R