This happened to a very good client of mine while I was in his office. I asked what the shouting was about after the guy slammed the phone down.
Someone called up to ask about a product they'd recently bought and were having problems with
The boss of this company spent about 5 minutes on the phone talking them through a few things to try. Theh he said "didn't we set this up for you when you bought it?"
The caller said "Oh, I didn't buy it from you I bought it from xxx.com (cheap box shifter)"
Good Client, "Well why didn't you call them about it?"
Caller "They don't offer technical support like you guys"
Sound of phone being slammed and ear splitting scream from Good Client and far too many expletives to list.