The next time you build some kind of product (be it software, a physical thing, or even a physical place or system like the NYC subway), try this thought experiment:

Would this thing still be usable if I were incredibly drunk?

All kinds of interesting challenges for makers emerge from the user errors that happen when you're trashed:

Your vision is impaired. on top of that, you'll be a lot slower cognitively. it's hard to read things... especially small labels. Everything is bleary and I can hardly squint. It's such a huge effort so i'm not gonna bother reading them at all and just cross my fingers and hope it does what it looks like. It's good to make the interface as synaesthetic as possible. If I screwed up and forgot to enter something, instead of being like "Enter a date in the third input field labelled 'End Date'" just fucking highlight the thing. Otherwise i'm gonna be like "ok where the hell in this page full of form-fields is that one labelled oh shit i gotta read each of these ARGH FUCKIT RAGEQUIT) A heavy iron gate with a big flashing red sign making scary noises is more likely to make me stop and turn around than some unassuming wooden door with a little "Do Not Enter" placard that takes me five minutes to notice while I'm fiddling with the knob. The more sensory dimensions you hit someone with (color, movement, sound, etc), the more likely it'll register through the noise, and the faster they understand what the product is trying to say to them.

Your pointing and gesturing becomes a lot less precise. You stumble into the bathroom and slap blindly at the wall for the light switch instead of carefully pressing it with one finger. If it has one of those fine tuned brightness knobs? Useless. It's really got only two relevant states now: all the way on or all the way off. It's gonna be really hard to click that tiny little button. TV remote? Forget it. So, you should make control targets nice, fat, and generous. Leave no ambiguity, and make important features especially distinguished. See what exaggeration does...

Because when you're drunk, lots of things start to look the same. These two switches with the same color laying right next to each other? I'm definitely gonna grab the wrong one now. The N, Q, and R trains all have yellow circles for symbols and stop at the same platform ... I just might jump on the wrong one.

You stop thinking and just run on habit. If all the light switches in my house turn the lights ON when they're in the UP position, except the one in the bedroom, I'm definitely gonna fuck up.

So, anyway ... there are even more examples. Next time, just perform this test yourself. Have a few drinks and see if that thing isn't frustrating to use.