Of course,
it's much quicker to tag something than to check it. I was
working on an article earlier that I added references for the bits I
could find and added a {{fact}} tag to one bit I couldn't verify,
although it seems right - the hope is that someone else comes across
it who can check it.
If you looked for a reference and couldn't find one, I think it would
have been better to remove the statement, rather than just tag it.
The "although it seems right" part is the important bit. I think it's
appropriate to leave in unsourced stuff that isn't controversial and one
is either reasonably sure is correct or at least one realizes that they
don't know enough in that subject area to have the faintest idea whether
it's correct. This is a collaborative work-in-progress, it's not
necessary for any one editor to do all the work on an article or to
leave an article in "perfect" condition at any given time.