Adjacent faces are separated on the UV map I have a truncated cone, open on both sides, laced near the top, with rims/curves along the top and bottom edges:

However, whenever I unwrap the object using the cylinder method (which seems to give the best layout solution overall), several faces that are adjacent on the model appear on the other side from their counterparts on the UV map:

This happened to the faces near the marked seam, but removing the seam - having no seams at all - results in the exact same layout.
The model does not have any modifier.
Does anyone know why this happens and how I can prevent it, or change this behaviour during unwrapping?


Additional images of the model (click for larger versions):
There's a problem in your model's topology.  Do this experiment.  In the UV editor, after you've done the cylinder projection, deselect everything and then carefully select the top row of vertices:

Since you've enabled UV Sync Selection, the same row of vertices shows up in the 3D view:

But if you zoom in to the active vertex you can see an issue:

It might be easier to see in edge select mode:

the path jumps from one edge loop to the other.  I don't know how to solve this directly, but my horrible work around is this:
select one of the vertical edge loops:

delete everything else:

add a screw modifier with the same number of steps:

and apply the screw modifier, in object mode.  The result is:

Notice that I didn't mark a seam, because I didn't know where you'd want it.
I don't honestly know what the topology issue is, but this &quot;fixes&quot; it.