Yeah, there's a chance any given crosspost will fail, based on how badly LJ is being DDoSed at any given time and whether or not DW can reach LJ's servers. The crosspost attempt will try up to 5 times, spread out over about an hour, to see if it's just a transient error; after that it will give up (assuming that the remote site is completely down and it doesn't make sense to keep trying) and give the user a permanent failure message. Because it's based on whether Dreamwidth's servers can reach LJ, not on whether you can reach LJ, there may be times when you can reach LJ but your crosspost fails, or your crosspost succeeeds but you can't reach LJ.

Basically, crossposting will be a little wonky until LJ's DDoS clears up.