Transactions sometimes failing validity check, sometimes not

[jargoman]

I'm signing transactions locally in C# using an object oriented model. I've had a number of failed transactions that were rejected by the server. The strange thing is sometimes they succeed, with orderes being placed on the orderbook or sometimes I get "error_exception":"fails local checks: Invalid signature", which is strange to me because the only value that would be different would be the fee or the sequence number. Why would a transaction fail one local check but later on succeed? I haven't been including a lastledgersequence because I want to test with minimum variables and add complexity slowly so I can test each change independently. 

The following are the response of two transactions that failed yesterday. 

Quote

{"error":"invalidTransaction","error_exception":"fails local checks: Invalid signature.","id":9,"request":{"command":"submit","id":9,"tx_blob":"1200072200000000240000168C64D40393DC7549F00000000000000000000000000042544300000000000A20B3C85F482532A9578DBB3950B85CA06594D165400000000665B67468400000000000000A7321027890161A61AE237CC1ECDCCB96339C11F2A7810A7A3524168DAD21F5B5B906A07448304402205A71EF5A634482837B073BB42B59E0555527E8DE53DEE719BFC97831B375AF8602204A6746232B08E99B1B0C96A325F7DD0223FC9F8A0A3354D7E4EB53EDF90AF76100008114779113FBAF345E2586C4CADC955247BF85DD8580"},"status":"error","type":"response"}

{"error":"invalidTransaction","error_exception":"fails local checks: Invalid signature.","id":9,"request":{"command":"submit","id":9,"tx_blob":"1200072200000000240000168C64D40393DC7549F00000000000000000000000000042544300000000000A20B3C85F482532A9578DBB3950B85CA06594D165400000000665B67468400000000000000A7321027890161A61AE237CC1ECDCCB96339C11F2A7810A7A3524168DAD21F5B5B906A074483045022000865F954AE8EC87BB72C133126C3327DB8C2DB451277808B34E769D5E011C28022100964F9187DC3F8126A9B7FDB3D4FCFD5C149A41B9A76A39D7DA5B946DC1093019008114779113FBAF345E2586C4CADC955247BF85DD8580"},"status":"error","type":"response"}
 

The following are two transactions  submitted today, one failed and the other didn't.

Quote

{"id":5,"result":{"engine_result":"tesSUCCESS","engine_result_code":0,"engine_result_message":"The transaction was applied. Only final in a validated ledger.","tx_blob":"1200072200000000240000168C64D40393DC7549F00000000000000000000000000042544300000000000A20B3C85F482532A9578DBB3950B85CA06594D165400000000665B67468400000000000000A7321027890161A61AE237CC1ECDCCB96339C11F2A7810A7A3524168DAD21F5B5B906A074483046022100949052FA51265FC0A59941D12B54F365606DF710D0A7D63CC7A5A4A2798DD777022100856CC8015DBD8F74F9DAFDB67FB0FDCF84F15CCF3AF0767DDFAB619CF0E5F3D58114779113FBAF345E2586C4CADC955247BF85DD8580","tx_json":{"Account":"rBuDDpdVBt57JbyfXbs8gjWvp4ScKssHzx","Fee":"10","Flags":0,"Sequence":5772,"SigningPubKey":"027890161A61AE237CC1ECDCCB96339C11F2A7810A7A3524168DAD21F5B5B906A0","TakerGets":"107329140","TakerPays":{"currency":"BTC","issuer":"rvYAfWj5gh67oV6fW32ZzP3Aw4Eubs59B","value":"0.01007"},"TransactionType":"OfferCreate","TxnSignature":"3046022100949052FA51265FC0A59941D12B54
F365606DF710D0A7D63CC7A5A4A2798DD777022100856CC8015DBD8F74F9DAFDB67FB0FDCF84F15CCF3AF0767DDFAB619CF0E5F3D5","hash":"AC8189BB6CD5744E5FE01713864E14AF8F44DAFCEC486B6743C65FCC74CE16F7"}},"status":"success","type":"response"}

Sending message :
{"id":9,"command":"submit","tx_blob":"1200072200000000240000168D64D40393DC7549F00000000000000000000000000042544300000000000A20B3C85F482532A9578DBB3950B85CA06594D165400000000669CD2B684000000000002A3A7321027890161A61AE237CC1ECDCCB96339C11F2A7810A7A3524168DAD21F5B5B906A074483045022100B5041676E0EF181FE7DE4F82D77521F24BA1FD8DEF30D392E3E79893742EF7040220733D4A39BC73FED17657C1DDC886F3829A9502DC53BD7F8E14DE7DF51DA01282008114779113FBAF345E2586C4CADC955247BF85DD8580"}

NetworkInterface : event websocket.MessageReceived : begin
Response from server wss://s1.ripple.com:443:
{"error":"invalidTransaction","error_exception":"fails local checks: Invalid signature.","id":9,"request":{"command":"submit","id":9,"tx_blob":"1200072200000000240000168D64D40393DC7549F00000000000000000000000000042544300000000000A20B3C85F482532A9578DBB3950B85CA06594D165400000000669CD2B684000000000002A3A7321027890161A61AE237CC1ECDCCB96339C11F2A7810A7A3524168DAD21F5B5B906A074483045022100B5041676E0EF181FE7DE4F82D77521F24BA1FD8DEF30D392E3E79893742EF7040220733D4A39BC73FED17657C1DDC886F3829A9502DC53BD7F8E14DE7DF51DA01282008114779113FBAF345E2586C4CADC955247BF85DD8580"},"status":"error","type":"response"}

Are my signatures invalid? Is the fee changing on me? Is it because I haven't included a last ledger sequence?