Usually we sent software updates on a floppy disc in the post.
It's 09:00 on April 1st 1979.
We sent some customers a fax. It told them that we were speeding-up the delivery of the software updates by using fax. Please cut-out the floppy disc below round the dotted line and follow the usual procedure to install the update.
At 15:45 we get a phone call. Tearful customer is upset. Our software update is stuck in her PC and it doesn't work and she can't get it out and she'll get in trouble and she doesn't know what to do and and and.....
Morals of the story?
1) Some customers will believe anything
2) Never ever assume customers will have any wits
3) Don't play April Fools games with customers.