I'm an MRI technician. A client came in for a scan, and we instructed her to take off her metal bracelet before her scan.
Client: This isn't metal - it's silver.
Client: What the hell! You overcharged me!
Me: I sent an invoice for what we agreed on.
Client: Bulls***. I'm looking at a bill for over four thousand dollars and we agreed on $500.
Me: Yeah, I sent an invoice for $500.
Client: Well then what the hell am I looking at?
Turns out it was his credit card bill.
Client: The site is in color... I didn't ask for that.
Me: Well, no, but it's in line with your branding. Is it wrong?
Client: It looks good, but I will NOT pay for the color. If it costs anymore, I want this site in Black and White - and don't you DARE try to charge me more.
I wrote an ad campaign for a client targetting millennials back when that was a thing. It was personable and authentic, and a big success when we ran it. The client got loads and loads of clickthroughs and I even won a local ad award for it.
After the first week, though, when the campaign was already a big success:
Client: You know, you should really make your ads more professional. This was way too casual. Just a little tip for the future.
I was flabbergasted. I'd just done a REALLY GOOD JOB and the client was still trying to give me career/life advice. Obviously he'd missed the point of the campaign, but IT WAS WORKING. You'd think he'd shut up at that point.
I was working with a client who had, and I quote, "decades of experience working with designers" and "loved to keep up with new technology."
Me: Would you mind sending me PNGs rather than JPGs? I could use the transparent background.
Client: What's a PNG? I've never heard of it. Is that a Photoshop thing or something?
Yeah, you're really up to date.