You don't have to go over the bridge with the barricade to get into the GDI base by BAR. If you are on the outside of the GDI base, looking at the BAR, you can climb the rocks to the left of the bridge and then drop down into the river and swim to the side of the bar. Not sure if it is a map glitch, but it definitely works.
Also, on WALLS there is a map glitch that allows NOD to walk up the map to get into the GDI base instead of having to go through the main entrance or the tunnel.
Wrong map. You're thinking of Gold Rush.
On Field, as far as I know there's no way for Nod to hop behind the bar, although GDI players can walk up the cliff above the back of bar which is definitely a map glitch (not gamebreaking or abuseable though).