EGR vacuum lines
Our intake manifolds are exposed to a pretty great deal of pressure through many on off cycles of varying degree gradually backing lines off. I would recomend spraying a bit of cheap ultra hold hair spray on the opposing nipple and then doubling up with an appropriately sized zip tie to secure the line. I think every line has popped off of my intake manifold at one point or another before I took the necessary steps. Aquanet and zipties are like pb&j they go so well together 
