I've only seen one carpet of doom from an AI. Diety continents map, and the other continent was a true carpet. So I declared war, and the entire carpet slid out to sea. Fortunately, I had more destroyers than the AI, so I sunk most of it before it reached me.
As for worker traffic jams, those of you who claim that they can't happen ... have you ever gone conquering on large (or huge) maps? When you're capturing cities fast enough that your national border is advancing an average of three or four hexes forward per turn, then you need a *lot* of workers to fix the mess the AI made of the terrain. (All those farms ... shudder...). I often have a three- or four-deep wall of workers advancing across the continent. That's a pain to manage. It's not uncommon for a worker to finish a job, and be unable to move anywhere because all nearby tiles already have a worker.