If you try to open a locked file with various editors you get interesting results:
windows notepad uncomplaining opens the locked file displaying its contents.
notepad++ opens (?) the locked file displaying no contents (empty buffer)
pspad doesn't open the file, instead, pop-ups an error dialog saying "Cannot open file xxx"
Now if you do some editing to the opened (yet locked) file and then chose to save,
more interesting things happen:
windows notepad informs that "the file is used by an another process", refuses to save and
offers the ability of [save as..]
notepad++ unfortunately does nasty things here: silently proceed and "saves" the file which means truncates everything to zero! (that is, not only destroys the old contents but also throws the newly edited contents into the black-hole of non-existence..)
I would say that the above np++ 's behavior is "a serious bug" but not before let you know and possibly confirm the problem.
P.S.: To my humble opinion, the most correct handling of the described situation is those of pspad.
P.S.2.: Nevertheless, np++ is my editor of choice and I'll grab the chance to say:
"Thank you very much for this great editor" ;-)