Since the LowS enforcement, we get messages like this with -debug=mempoolrej and 'sendrawtransaction':

2015-10-21 08:44:56 65fcd08dbfa3e806f62b7eb1e9a9e21070e88e72f0b41ed4d83afa4e71b63d50 from peer=58 was not accepted: non-mandatory-script-verify-flag (No error) (code 64)

The message could at least mention what non-mandatory script-verify flag. We know, but other users may not.