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What does it mean to have conflicts between runtimes?

I read this in Docker for developers. Can someone explains what this means and give an example? Thanks!
Another similar problem is when you want to host App 3 on the same server, but App 3 uses the Node.JS runtime together with a package that, when installed, changes a dependency used by the PHP runtime. Conflicts between runtimes happen often, so you’ve probably faced that problem already.