There's no benefit to a scary stack trace here:

m30999| 2014-10-30T07:01:34.912 0000 I - [conn1] Assertion: 10412:moveAndCommit failed: { cause:

{ ok: 0.0, errmsg: "can't accept new chunks because there are still 1 deletes from previous migration" }

, ok: 0.0, errmsg: "moveChunk failed to engage TO-shard in the data transfer: can't accept new chunks because there are still 1 deletes from previous migration" }
m30999| 2014-10-30T07:01:34.921 0000 I CONTROL [conn1]
m30999| 0x898a313 0x892af68 0x890ce78 0x890d546 0x871f84e 0x87227ad 0x8748cd9 0x874a300 0x874c173 0x875c06f 0x88365b1 0x878518f 0x8854deb 0x8834d12 0x82f44c8 0x893a707 0x789939 0x2d07ae
m30999| ----- BEGIN BACKTRACE -----
m30999| mongos(mongo::printStackTrace(std::ostream&) 0x33) [0x898a313]
m30999| mongos(mongo::logContext(char const*) 0xC8) [0x892af68]
m30999| mongos(mongo::msgasserted(int, char const*) 0xF8) [0x890ce78]
m30999| mongos( 0x8C5546) [0x890d546]
m30999| mongos( 0x6D784E) [0x871f84e]
m30999| mongos(mongo::Chunk::splitIfShould(long) const 0xCFD) [0x87227ad]
m30999| mongos( 0x700CD9) [0x8748cd9]
m30999| mongos(mongo::ClusterWriter::shardWrite(mongo::BatchedCommandRequest const&, mongo::BatchedCommandResponse*) 0x1F0) [0x874a300]
m30999| mongos(mongo::ClusterWriter::write(mongo::BatchedCommandRequest const&, mongo::BatchedCommandResponse*) 0xEA3) [0x874c173]
m30999| mongos(mongo::ClusterWriteCmd::run(mongo::OperationContext*, std::string const&, mongo::BSONObj&, int, std::string&, mongo::BSONObjBuilder&, bool) 0x47F) [0x875c06f]
m30999| mongos(mongo::Command::execCommandClientBasic(mongo::OperationContext*, mongo::Command*, mongo::ClientBasic&, int, char const*, mongo::BSONObj&, mongo::BSONObjBuilder&, bool) 0x531) [0x88365b1]
m30999| mongos(mongo::Command::runAgainstRegistered(char const*, mongo::BSONObj&, mongo::BSONObjBuilder&, int) 0x4CF) [0x878518f]
m30999| mongos(mongo::Strategy::clientCommandOp(mongo::Request&) 0x39B) [0x8854deb]
m30999| mongos(mongo::Request::process(int) 0x7E2) [0x8834d12]
m30999| mongos(mongo::ShardedMessageHandler::process(mongo::Message&, mongo::AbstractMessagingPort*, mongo::LastError*) 0x78) [0x82f44c8]
m30999| mongos(mongo::PortMessageServer::handleIncomingMsg(void*) 0x4B7) [0x893a707]
m30999| libpthread.so.0( 0x5939) [0x789939]
m30999| libc.so.6(clone 0x5E) [0x2d07ae]
m30999| ----- END BACKTRACE -----