This appears to have been fixed in 1.11.1. I wouldn't be surprised if r55800 (which fixed a similar issue with creating units with Lua) fixed the problem.

From further investigations, it appears that:
- when "A New land" game is hosted on a MP server, an observer can observe and replay the game without OOS on recruit
- if the host is saving the game and load it on local with replay, the replay is creating the SYNC error ass described in the original post.

When running BFW with --log-debug=replay,random, the following log is created when locally loading the save with replay:
20121112 21:29:11 debug random: Seeded random with 279659955 with 0 calls, pool is now at 279659955 20121112 21:29:13 debug replay: in replay_controller::init()... 20121112 21:29:13 debug replay: initializing replay-display... 2660 20121112 21:29:13 debug replay: done initializing replay-display... 2661 20121112 21:29:17 debug replay: Status turn number: 1 20121112 21:29:17 debug replay: Replay_Controller turn number: 1 20121112 21:29:17 debug replay: Player number: 1 20121112 21:29:17 debug replay: doing replay 1 20121112 21:29:17 info replay: up to replay action 1/27 20121112 21:29:17 debug replay: Replay data: [start] [/start] 20121112 21:29:17 info replay: up to replay action 2/27 20121112 21:29:17 debug replay: Replay data: sent = [init_side] [/init_side] 20121112 21:29:17 info replay: up to replay action 3/27 20121112 21:29:17 debug replay: Replay data: sent = [recruit] value = 0 x = 28 y = 26 [from] x = 29 y = 27 [/from] [/recruit] [random] value = 15563 [/random] [random] value = 12503 [/random] [random] value = 12552 [/random] [random] value = 31107 [/random] [random] value = 31711 [/random] [random] value = 15849 [/random] [random] value = 3600 [/random] [random] value = 9344 [/random] [random] value = 26031 [/random] [random] value = 17528 [/random] [random] value = 526 [/random] [random] value = 3307 [/random] [random] value = 30663 [results] checksum = ldl0N<-,N<5Tbo81Kr4jn0s1a1ql79,2A+0eHk->j6Yf<9nEbqob,e2<DBmzDMMBGssFHA.DGDmwvsMGHRGgCtKwKy3bp8q,LReMEtKAIHnStwPgDM6v7LKKvNFuMyqp [/results] [/random] 20121112 21:29:17 debug random: pulled user random 3152900227 for call 1 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 15341 vs 15563 20121112 21:29:17 info random: get_random() returning 15563 20121112 21:29:17 debug random: pulled user random 2744870768 for call 1 with seed 279659955 20121112 21:29:17 debug random: pulled user random 1934580457 for call 2 with seed 279659955 20121112 21:29:17 debug random: pulled user random 2763194880 for call 2 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 9395 vs 12503 20121112 21:29:17 info random: get_random() returning 12503 20121112 21:29:17 debug random: pulled user random 919865913 for call 3 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 14036 vs 12552 20121112 21:29:17 info random: get_random() returning 12552 20121112 21:29:17 debug random: pulled user random 1417406590 for call 4 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 21627 vs 31107 20121112 21:29:17 info random: get_random() returning 31107 20121112 21:29:17 debug random: pulled user random 2299302367 for call 5 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 2316 vs 31711 20121112 21:29:17 info random: get_random() returning 31711 20121112 21:29:17 debug random: pulled user random 2688957996 for call 6 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 8262 vs 15849 20121112 21:29:17 info random: get_random() returning 15849 20121112 21:29:17 debug random: pulled user random 3873575157 for call 7 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 26338 vs 3600 20121112 21:29:17 info random: get_random() returning 3600 20121112 21:29:17 debug random: pulled user random 2179600010 for call 8 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 490 vs 9344 20121112 21:29:17 info random: get_random() returning 9344 20121112 21:29:17 debug random: pulled user random 3922881787 for call 9 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 27090 vs 26031 20121112 21:29:17 info random: get_random() returning 26031 20121112 21:29:17 debug random: pulled user random 2968457496 for call 10 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 12527 vs 17528 20121112 21:29:17 info random: get_random() returning 17528 20121112 21:29:17 debug random: pulled user random 2106574705 for call 11 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 32143 vs 526 20121112 21:29:17 info random: get_random() returning 526 20121112 21:29:17 debug random: pulled user random 929204566 for call 12 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 14178 vs 3307 20121112 21:29:17 info random: get_random() returning 3307 20121112 21:29:17 debug random: pulled user random 2840649175 for call 13 with seed 806898482 20121112 21:29:17 info random: Random number mismatch (nocheck), mine 10576 vs 30663 20121112 21:29:17 info random: get_random() returning 30663 20121112 21:29:17 error engine: SYNC: In recruit Mage: has checksum ldl0N<-,N<5Tbo81Kr9jn0s1a1qlf9,2A+0eHk->j6Zm<9nEbqob,e2<DBmzDMMBGssFHA.DGDmwvsMGHRGhvtKwKy3bp8q,LReMEtKAIHnStwPgDM6v7LKKvNFuMyqp while datasource has checksum ldl0N<-,N<5Tbo81Kr4jn0s1a1ql79,2A+0eHk->j6Yf<9nEbqob,e2<DBmzDMMBGssFHA.DGDmwvsMGHRGgCtKwKy3bp8q,LReMEtKAIHnStwPgDM6v7LKKvNFuMyqp

We see that two different random seeds are used alternatively: 806898482 and 279659955.

Running the same mode with observer on MP server shows that only seed 279659955 is used and no OOS is raised.