In a transfer which include several files, sometimes I may want to removed files with incomplete pieces, or just unwant files. However, this action sometimes hurt the main file, leaving 99% instead of 100% of the main file I have downloaded.
I have encountered this many times and, sadly, I cannot see the lost piece anymore from my peers!!!

simply unselect the files you don't want before downloading.

once it is downloaded i don't think this can be possible

usually I do unselect before I download. But it may sometimes download unwanted files nevertheless. Sometimes, it is because some pieces were missing from a file and I don't want to keep the "broken" file. When I delete those incomplete files, the Main file may damage.

normally at least wxith utorrent) the file are complete on the disk but it's seen by the ctorrent client as incomplete because one piece contains the end the kept file but also the beginning of a deleted one. and since you only got a part of that piece, the file is seen as incomplete

Today, I tried the other way round.

I stopped the transfer, unselected files I don't want in the package, deleted unwanted files using Windows File Manager, "Force Check"ed the transfer.

Hay! the main file is incomplete again !!!

What's going on? Is the file allocation table cross-linked or something?

Do not delete those files manually, do it with Tixati instead. Select unwanted files in Tixati and set Priority to Off. After that there will be available additional Delete option in context menu, so you can remove those files with Tixati safely. It won't damage other files from a transfer, Tixati will move crust pieces from deleted files to the incomplete-pieces directory.

Thank you all. I have tried all you suggested. Main file is broken in random cases.
*Sigh*

Oh no, removing wanted pieces is driving me crazy. /..\