micky wrote:
Can you damage a case fan by vacuuming the air vents from the outside?

(I have a Dell Optiplex 775 with the original fan. )

It seems more than safe to me, but I happened to read the manual for a
Noctua fan and it says not to use a vacuum cleaner or it may apply
"excessive force to the fan". I find that hard to believe. I've pushed
on the vanes of other 3, 4, and 5" fans and they don't bend at all with
the kind of force vacuum cleaners exert. In fact I think they would be
hard to break. Are Noctua fans more fragile than others?

Spinning a fan at 26,000 rpm is not good for the fan.
They tend to violently explode.
(Don't ask how I know this.)
Always hold the blades or stick something in to hold the blades in place.