I'm currently brainstorming on my Lemmings Multiplayer Online Game and solving problems in my head already.
Now the next problem must have been encountered by other game-programmers too:

How do you synchronize game-mechanics when some people are playing a game together?
Let's take a lemmings-level as example. 2 players have control over their own lemmings in the same level. Now we know lemmings can change the world they are in, by building digging etc.
I can imagine problems because of network-latency (lag) for example: Player A starts digging at frame 100, Player B starts building at the same place at frame 100, but because of lag does not yet see what player A is doing.
How to handle these kind of problems? The Lemmings game needs a high degree of precision on timing and pixels.