At worlds there was a point in the pivotal and very exciting final championship where the finer point of entry/exit of terrain came up.
Simply if you're already on contact with terrain at the start of your activation, you can't just march or shift into it. See 18 and 81 in the RRG especially focus on the collision bit.
Perhaps I might be speaking into the void but I think this is something that FFG should reconsider for the purposes of the rules. I think you should be able to shift into or march into terrain, ie willfully collide with terrain you are already in contact with. Yes this might trigger more game effects like those for the bone pit, but I think in the grand scheme the game would be in a better place. As it is now, a player isn't left with a lot of options that will generally be slow/painful for a turn or two before the unit might be able to recover. In an 8 turn game, that's a fairly high price to pay.
On the positive side, for this kind of change, I think it introduces more strategy around terrain and it's use. I think that is a VERY good thing. It adds to Seasoned Pathfinder & Scouts and certainly could be value to similar abilities for future factions/units and their upgrades.
Something to thinking about / debate.