As we all know not all browsers like to behave the same. Also many clients just click the Big Blue E and do not know what browser they use. Such and such is broken in IE, Firefox, etc. Obviously with the rise of the ever green browsers this is not always an issue but when it is http://yourbrowser.is/ is your best friend. It allows you to send your client a specific link to send you important information about your clients browser.
This is amazing because the issue is not always the browser it could be the way that the OS handles something. This way you can use a tool like Browserstack to get the closest configuration as possible to the browser the issue happened in. This will definitely help when tracking down those fun ghost in the machine type bugs impossible to replicate even though you can physically see it happening on your clients machine. So I suggest you head over there right now and try it out.