1. A persistent, harmless, fan.
A person who is famous within an organisation or department because they keep popping up, 'helping', suggesting ideas and generally showing an unhealthy interest in whatever the organisation does. This person has no official connection with the company and yet seems to be more loyal to it than its own employees.
A good serial pest will have somehow managed to discover your the boss's direct email address.
2. A person who is addicted to pulling public pranks and is regularly in the press for doing so.
Media Mogul: We need a new regular feature in the Entertainments Section of the mag. Got any ideas?
Magazine Editor: One of my serial pests suggested something yesterday. It might just fit. I'll dig it out.
Employee 1: I bumped into 'Hairy Man' this morning, you know, the serial pest. He said he wanted to come to the packaging tradeshow with us and help out on the exhibition stand.
Employee 2: Great. He can take my place.
Employee 1: Ha! Very funny.