Usually it means that you are trying to program some behavior based on what sort of browser is running.
If browser = "ie" then do_something_cool; else do_something_different;
That sort of thing is not going to end well.
If you're just tracking what browsers users appear to be using to access your site for statistical purposes then you probably aren't actively creating problem code but you should keep in mind that browsers lie about what the user agent is. (They sort of have to due to IE's long and sordid history of malfeasance.) And even if the browser itself isn't lying the user might be because the user agent is user settable.