If you poke yourself in the eye with the corner of your iPad, that's likely your fault.
If you swallow your Samsung Galaxy S5 during a drunken dare, you surely can't blame anyone else.
But what if you buy Google Glass, wear it and discover you get headaches. Whose fault is it then?
After Technically Incorrect raised the issue of Google Glass and headaches earlier this year, Google responded by saying that it would be silly to wear Glass for hours on end.
Yet when people buy new, shiny, expensive things they like to play with them to excess. People aren't as smart as Google engineers.
This week, Google's optometrist, Dr. Eli Peli, reiterated to Betabeat that wearing Glass for the first time is no different than wearing any other glasses for the first time.
If you keep looking up at the display, he said, this may cause eyestrain. If you wear the product for hours on end, this may cause eyestrain too.
He explained it like this: "You're on one leg [or the other] as you walk, but try to stand on one leg for a long time and you'll feel tension, because you're not using it how it's normally used. If you're looking at the Glass for a minute, you're holding it there for 60 times longer than normal."
Sometimes, eyestrain is associated with headaches, although in the case of one of the first Google Glass Explorers, Chris Barrett of PRServe, he believes it's more than that. He says he suffered headaches so bad that he stopped using Glass almost entirely.
He's limited his usage to just 30 minutes so far this year. However, he told me: "Using Glass for even a couple minutes in a row does strain my eye. The prism is so far out of my viewing path that it is a pain to look up and to the right. It's not a normal eye motion."
I asked Google whether it wasn't time to at least consider putting warnings on the product, especially as it's now being sold to the (wealthy) general public.
The company wouldn't answer that specific question. Instead, it insisted that the product was designed for "micro-interactions," as if this had always been obvious.
A Google spokesperson told me: "As we note in our Help Center, Glass is designed for micro-interactions, not for staring into the screen, watching Friday night movie marathons or reading 'War and Peace.'"
Some will feel this makes absolute sense. It's just the same as people buying a coffee and then being shocked that it's hot. Others will say that this is yet another case of engineers demanding that real humans adjust to the engineering, rather than the engineering adjusting to the humans.
For his part, Barrett readily admits he may have overused Glass. But he does believe Google should do more to help new users.
He told me: "I think Google needs to educate its users about Glass even more. I don't think there will be a cigarette style warning on a Glass box ever -- Glass is not going to kill you. It's not going to give you cancer from long-term use. I don't think there needs to be a skull and crossbones style warning on the box. Maybe just more of a common sense warning."
Some might say, however that common sense and Google don't always see eye to eye. The company has clearly been surprised by some of the negative reaction around the Glass design and the way some users have behaved like Glassholes.
Barrett foresees a much bigger headache if the product was suddenly mass-produced: "Google would have a problem on their hands if they sold millions of Glass at once and tens of thousands of people complained of headaches or eyestrain at a launch."