As I posted in another comment, the problem you describe exists for this person regardless of whether it involves an ad or not. The entire UI for this app is a serious problem with a person of these limitations.
I fail to see how going to an ad page is a worse offense than going to Drafts as opposed to Inbox. Either way the person in question has to correct the mistake. Expensive in what way? Money, time, embarrassment? I don't see the difference.
The ad is between Compose and Inbox, so what? Are you suggesting that Yahoo somehow benefits from an accidental click on this ad? What's the purpose of them putting it there? Could it be possible they have it there because they consider that you are more likely to see it there and not that you'll accidentally click on it?
And again, what's the optimum distance that's acceptable? Fifty pixels? One hundred? I would have to assume that if this is the problem then it is a potential problem for nearly every website and application ever made. The reason this bothers me is because people are saying the burden is on Yahoo to fix this when the problem does not necessarily lay with them, but other factors are in play they have no control over. Some people are even suggesting nefarious reasons for this ad placement to "trick" people into clicking on it. For what benefit?
I too sometimes click on something other than what I intended. Most of the time I don't blame the app, I blame myself. There are times that people will employ tricks to get you to do what they want, that's when you complain; this is not a good example of this.
I fail to see how going to an ad page is a worse offense than going to Drafts as opposed to Inbox. Either way the person in question has to correct the mistake. Expensive in what way? Money, time, embarrassment? I don't see the difference.
At first it's going to be confusion. It's the difference between quickly being able to understand the mistake (because the whole interface is still on the screen, with the pointer maybe in the same position) vs. suddenly being on a completely different site. Keep in mind that this is a missed link, so the user may not immediately know what has been clicked.
After the guy has been using Yahoo Mail for a while and the peril of that ad is well understood, it's just going to become really really annoying, getting sent off-site, every time he hits it. And he will hit it.
Could it be possible they have it there because they consider that you are more likely to see it there and not that you'll accidentally click on it?
Sure. But is that the best choice? That's the question. Like you say, even fully able-bodied people sometimes click the wrong link. It seems like a good idea to take that into consideration when placing what may be the two most trafficked buttons within your interface. I'm not assuming nefarious purposes on Yahoo's part.
I can't say what the optimum vertical height of a button should be for a shaky person. Gmail serves my purposes fairly well without making many design compromises.
I fail to see how going to an ad page is a worse offense than going to Drafts as opposed to Inbox. Either way the person in question has to correct the mistake. Expensive in what way? Money, time, embarrassment? I don't see the difference.
The ad is between Compose and Inbox, so what? Are you suggesting that Yahoo somehow benefits from an accidental click on this ad? What's the purpose of them putting it there? Could it be possible they have it there because they consider that you are more likely to see it there and not that you'll accidentally click on it?
And again, what's the optimum distance that's acceptable? Fifty pixels? One hundred? I would have to assume that if this is the problem then it is a potential problem for nearly every website and application ever made. The reason this bothers me is because people are saying the burden is on Yahoo to fix this when the problem does not necessarily lay with them, but other factors are in play they have no control over. Some people are even suggesting nefarious reasons for this ad placement to "trick" people into clicking on it. For what benefit?
I too sometimes click on something other than what I intended. Most of the time I don't blame the app, I blame myself. There are times that people will employ tricks to get you to do what they want, that's when you complain; this is not a good example of this.