The other scenarios -- restore my iPhone, delete my recycle bin, and ate gluten -- I could see happening.

But accidentally build a shelf? Come on!

Aha! Reddit with the explanation:

If anyone came here looking for a potential explanation it is possible the original searchers are referring to creating a tool shelf in a software package and can't get rid of it. Basically the equivalent of a bookmarks bar on a web browser.

Sounds... frustrating.

More: http://pandawhale.com/post/14956/why-must-life-be-so-hard-homer-simpson-gif

Hmm, I thought this meant someone bought a dresser from Ikea and accidentally built a bookshelf.

Or trying to build a desk and accidentally installing a shelf on it:

But frankly, that shelf seems rather useful. In fact, I could use one of those.

More: http://pandawhale.com/post/7019/ikea-hackers-home-office-in-a-box

Reddit says key to accidentally building a shelf is to throw a bunch of lumber at the ground... and miss!

You know, like Douglas Adams says:

There is an art, it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss. Pick a nice day, [The Hitchhiker's Guide to the Galaxy] suggests, and try it.

The first part is easy. All it requires is simply the ability to throw yourself forward with all your weight, and the willingness not to mind that it's going to hurt.

That is, it's going to hurt if you fail to miss the ground. Most people fail to miss the ground, and if they are really trying properly, the likelihood is that they will fail to miss it fairly hard.

Clearly, it is the second part, the missing, which presents the difficulties.

One problem is that you have to miss the ground accidentally. It's no good deliberately intending to miss the ground because you won't. You have to have your attention suddenly distracted by something else when you're halfway there, so that you are no longer thinking about falling, or about the ground, or about how much it's going to hurt if you fail to miss it.

It is notoriously difficult to prize your attention away from these three things during the split second you have at your disposal. Hence most people's failure, and their eventual disillusionment with this exhilarating and spectacular sport.

If, however, you are lucky enough to have your attention momentarily distracted at the crucial moment by, say, a gorgeous pair of legs (tentacles, pseudopodia, according to phyllum and/or personal inclination) or a bomb going off in your vicinty, or by suddenly spotting an extremely rare species of beetle crawling along a nearby twig, then in your astonishment you will miss the ground completely and remain bobbing just a few inches above it in what might seem to be a slightly foolish manner.

This is a moment for superb and delicate concentration. Bob and float, float and bob. Ignore all consideration of your own weight simply let yourself waft higher. Do not listen to what anybody says to you at this point because they are unlikely to say anything helpful. They are most likely to say something along the lines of "Good God, you can't possibly be flying!" It is vitally important not to believe them or they will suddenly be right.

Waft higher and higher. Try a few swoops, gentle ones at first, then drift above the treetops breathing regularly.

DO NOT WAVE AT ANYBODY.

When you have done this a few times you will find the moment of distraction rapidly easier and easier to achieve.

You will then learn all sorts of things about how to control your flight, your speed, your maneuverability, and the trick usually lies in not thinking too hard about whatever you want to do, but just allowing it to happen as if it were going to anyway.

You will also learn about how to land properly, which is something you will almost certainly screw up, and screw up badly, on your first attempt.

There are private clubs you can join which help you achieve the all-important moment of distraction. They hire people with surprising bodies or opinions to leap out from behind bushes and exhibit and/or explain them at the critical moments. Few genuine hitchhikers will be able to afford to join these clubs, but some may be able to get temporary employment at them. 

-Douglas Adams