/proc has surely killed some user apps. If you were expected to see a fixed number of lines, a fixed number of columns or the fields to mean the same thing, you're in for suffering. At least I know of meminfo changes, stuff in /proc/net...

But to be honest, so have ifconfig, hostname, df...

At least in procfs they add things (columns, lines) so apart from meminfo I don't know of changing the meaning of fields.