I had a situation a few years ago where someone wrote some letters at a siyum sefer torah without saying leshaim. I was out of the room at the time and the guy just sort of did it on his own without asking anyone. We later could not remember which letters he did so we didn't have the option of erasing and rewriting.
I subsequently called a number of Rabbonim to clarify what to do. Teshuvios ranged from those who said to erase and rewrite the entire final omud except the shaimos (we left the whole omud for this siyum ,it was a big one) to those who said it was OK since the outline was already there and that was written lishmah.
In the end the morah deasrah of the shule followed the majority opinion that it was OK since the outline was there and there is no shiur of the uvai haos so therefore the letter was kosher even before.
However this incident still nags at me and I still remain a little uneasy about it. I would appreciate anyone's imput or similar experiences / teshuvos.