Is it really a prank if the victim doesn’t realize there’s a prank going on?

One of my friends pranked a colleague by surreptitiously putting a program in the Startup group. This program opened a network socket and awaited further instructions.

Things you could tell the program to do:

• Create a thread and put it into a 100% CPU loop.
• Allocate a megabyte of memory (hey, that was a lot back in the day) and go into a loop accessing every byte of memory over and over again.
• Display a message on the screen.
• Play a beep.
• Perhaps some other pranky things, I forget.

My friend told the program to create a thread and peg the CPU, and his colleague didn’t notice.

Create a second thread.

Still no reaction.

Allocate a megabyte of memory.

Two megabytes.

Four megabytes.

Still nothing.

Beep a few times for no apparent reason.

No reaction.

My friend now decided to pull out all the stops. Create eight threads, each in a CPU spin loop. Allocate another 16 megabytes of memory. Beep five times in a row. Display a message box with a nonsensical error message. (I forget exactly what it was. Let’s say it’s “Printer out of bamboo. Get more bamboo. MOAR!”)

My friend’s colleague shrugged and dismissed the error message.

I think my friend had to display a message box that said “Look behind you” before his colleague finally clued in that he was being pranked.

So you tell me. Was this a successful prank or not, if the victim didn’t even realize that he was being pranked?