What is this folder ( DocumentRevisions-V100-bad-1) ?

Hi

one day i was working on Preview ( PDF ) i made some changes and saved a copy. then i wanted to close the original file, a small window comes up asks for ( Revert Changes , Cancel or Save ) i chosen ( Revert changes ) but an error came up saying ( the operation could not be completed ). i forced quit the app. restarts my MBP. Then i went to check on the .DocumentRevisions-V100 folder. suddenly i found another one named ( .DocumentRevisions-V100-bad-1 ) what is that folder ? and how to prevent this problem from coming again.

know everything is normal on my Mac. but all the old versions i believe are in .DocumentRevisions-V100-bad-1.

thank you

MACBOOK PRO

Retina, 15-inch, Late 2013

OS X 10.9.5