<p>As I use the web, I regularly get runtime errors (usually javascript) being reported via popups. This can make for a really unsatisfying user experience on many otherwise excellent websites and also makes me wonder what functionality I am not getting access to.</p>

<p>Why is this such a common issue? Is this down to a lack of testing or is it browser compatibility issues? What can be done to minimise this kind of issue?</p>

<p>Incidently I don't have the 'Display a notification about every script error' checked.</p>

It's easy to make mistakes in JavaScript. Until recently with Aptana, there weren't many good tools for coding JavaScript. Not have the benefit of syntax checking or compilation, small typing error can make it into pages. Beyond that, there are time where code that works fine in one browser will cause an error in another. For example, I saw a jQuery plugin that didn't work in IE, but it was fine in every other browser. It turned out to be a variable declared without the 'var' keyword. Firefox and Safari were OK with that, IE wasn't, so it's possible that whoever wrote the code didn't even know it was a problem.