I had the problem with the Cessna 140. Turned out to be bad fuel vents
(caps in this case). I remember flying at night and looking up and
noticing the left tank was 100% empty. Apparently the plane figured out
how to get to the right tank on its own (this plane had a "both"
selector, unlike earlier C140's).

My current plane doesn't have a both. I"m not sure I would use a both
if I did have it. If for some reason I run out of gas (either my own
fault of a leak), I'd like to have a back up tank to switch to.