Client Keeps Seeing Outdated Version Through Browser?

I ran into this recently and it threw me for a loop.

A client kept saying that they were seeing the old version of a short video when viewed in their browser via a direct link. I checked it it was the newer version.

We ran through all the cache clearing stuff, relaunching the browser, rebooting her computer and even trying another browser but no luck; they kept seeing (and me hearing over the phone) the older version.

Snooping around (which means calling a bunch of friends) I found out that they too, were seeing the old version. This was insane. Until one friend hit some keys and like magic I heard the new version playing over the phone.

Me - "Okay, what's that worth to me you're thinking, right?"
So-Called Friend - "Yeah, I was thinking a free lunch. I like those."
Long pause...
Me - "Fine."
So-Called Friend - "Any place?"
Me - "Any place."
So-Called Friend - "Cool. Meet me at (his favorite place) for lunch and I'll tell you there."

Evil.

At lunch, he goes, "Put a question mark at the end of your url. It'll trick the ISP into serving up the right version and bypass their cache."

I tried it. It worked. The client's ISP sucks. So I billed my friend's free lunch to the client. Shhhh...