Have you ever tried to debug a site where a beginner web designer has loaded the page with scripts? There's a couple copies of jQuery in various versions, maybe a little Prototype or MooTools thrown in. We can laugh a bit, but many of us went through that stage where we just didn't really understand things well enough to know there was any problem with that. And is there? Aside from the bandwidth concerns, most libraries have steps you can take to ensure they play well with other libraries if need be. So what's the scoop? Do you ever use multiple libraries? Poll is in the sidebar.