Speaking in terms of real pop-up boxes, you might be surprised at how easy it is for people to ignore the content of even the most blaring, attention-grabbing error messages.

A typical computer user's reaction to a pop-up box is to immediately click whatever they think will make it go away, because a pop-up box is not a message to be understood but a distraction from what they're actually trying to accomplish. A more obnoxious pop-up box just increases the user's agitation to get rid of it.

As rationalists, we try hard to avoid falling into traps like these (I'm not sure if there's a name for the fallacy of ignoring information because it's annoying, but it's not exactly a high-utility strategy), but part of the way we should do that is to design systems that encourage good habits automatically.

I like Firefox's approach; when it wants you to choose between Yes or No on an important question ("Really install this unsigned plugin?"), it actually disables the buttons on the pop-up for the first 3 seconds. You see the pop-up box, your well-honed killer instinct kicks in and attempts to destroy it by mindlessly clicking on Yes so you can get back to work already... but that doesn't work, you're surprised, and that jolt out of complacency inspires you to actually read the message.

I suspect a "Hey, have you noticed that something has penetrated the skin of your left foot?" warning might benefit from having the same mechanism.