We have faced with following exception trying to read from DB, and we can not reproduce it once more. All read\write operations to Mongo DB are perfomed with C# MongoDB driver 1.8.3, Mongo DB version : 2.6.1.

ERROR Unable to translate bytes [AB] at index 106 from specified code page to Unicode.
Exception: System.Text.DecoderFallbackException
Message: Unable to translate bytes [AB] at index 106 from specified code page to Unicode.
Source: mscorlib
at System.Text.DecoderExceptionFallbackBuffer.Throw(Byte[] bytesUnknown, Int32 index)
at System.Text.DecoderExceptionFallbackBuffer.Fallback(Byte[] bytesUnknown, Int32 index)
at System.Text.DecoderFallbackBuffer.InternalFallback(Byte[] bytes, Byte* pBytes)
at System.Text.UTF8Encoding.GetCharCount(Byte* bytes, Int32 count, DecoderNLS baseDecoder)
at System.String.CreateStringFromEncoding(Byte* bytes, Int32 byteLength, Encoding encoding)
at System.Text.UTF8Encoding.GetString(Byte[] bytes, Int32 index, Int32 count)
at MongoDB.Bson.IO.BsonBuffer.DecodeUtf8String(UTF8Encoding encoding, Byte[] buffer, Int32 index, Int32 count)
at MongoDB.Bson.IO.BsonBuffer.ReadString(UTF8Encoding encoding)
at MongoDB.Bson.IO.BsonBinaryReader.ReadString()
at MongoDB.Bson.Serialization.Serializers.BsonStringSerializer.Deserialize(BsonReader bsonReader, Type nominalType, Type actualType, IBsonSerializationOptions options)
at MongoDB.Bson.Serialization.Serializers.BsonBaseSerializer.Deserialize(BsonReader bsonReader, Type nominalType, IBsonSerializationOptions options)
at MongoDB.Bson.Serialization.Serializers.BsonValueSerializer.Deserialize(BsonReader bsonReader, Type nominalType, Type actualType, IBsonSerializationOptions options)
at MongoDB.Bson.Serialization.Serializers.BsonBaseSerializer.Deserialize(BsonReader bsonReader, Type nominalType, IBsonSerializationOptions options)
at MongoDB.Bson.Serialization.Serializers.BsonDocumentSerializer.Deserialize(BsonReader bsonReader, Type nominalType, Type actualType, IBsonSerializationOptions options)
at MongoDB.Bson.Serialization.Serializers.BsonBaseSerializer.Deserialize(BsonReader bsonReader, Type nominalType, IBsonSerializationOptions options)
at MongoDB.Driver.Internal.MongoReplyMessage`1.ReadFrom(BsonBuffer buffer, IBsonSerializationOptions serializationOptions)
at MongoDB.Driver.Internal.MongoConnection.ReceiveMessage[TDocument](BsonBinaryReaderSettings readerSettings, IBsonSerializer serializer, IBsonSerializationOptions serializationOptions)
at MongoDB.Driver.Operations.QueryOperation`1.GetFirstBatch(IConnectionProvider connectionProvider)
at MongoDB.Driver.Operations.QueryOperation`1.<Execute>d__0.MoveNext()
at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
at MongoDB.Driver.MongoCollection.FindOneAs[TDocument](IMongoQuery query)
at MongoDB.Driver.MongoCollection.FindOneByIdAs[TDocument](BsonValue id)