It was this post that made me upload my pictures of the red squares I'd seen - specifically The_Gear asking for pictures. I intended to post here with a comment but was called away due to Halloween frivolity. So thanks Leo in WI for posting the link for me.
I first assumed it was just one terrain type, now I'm not so sure... I'm used to pink blobs meaning missing textures from other games (Civ4 mod-mods, where attack of the big pink blob is a common enough sight). But I've seen them all over, plains, forests and in nasty dead terrain. But without it naming the terrain-type in the tooltip they may all actually have been squares of just one or two terrain types.
On terrain types, I think flight lists terrains where it ignores the usual movement penalty, but that list is only half complete - ignoring burning lands for example.