I found this piece via Damien Katz. It is originally from a freebsd mailing list:

In the specific example involving the bike shed, the other vital
component is an atomic power-plant, I guess that illustrates the
age of the book.

Parkinson shows how you can go in to the board of directors and
get approval for building a multi-million or even billion dollar
atomic power plant, but if you want to build a bike shed you will
be tangled up in endless discussions.

Parkinson explains that this is because an atomic plant is so vast,
so expensive and so complicated that people cannot grasp it, and
rather than try, they fall back on the assumption that somebody
else checked all the details before it got this far. [...]

A bike shed on the other hand. Anyone can build one of those over
a weekend, and still have time to watch the game on TV. So no
matter how well prepared, no matter how reasonable you are with
your proposal, somebody will seize the chance to show that he is
doing his job, that he is paying attention, that he is here.

I don’t know how many times I had to endure pointless arguments in meetings. At least this explanation is less depressing than the one I had before about people just being plain stupid. People should be forced to read this mail before every single meeting.