Sonar is playing one MIDI bass note an octave higher than written.

UPDATE: There another strange thing going on in the same clip. I have a three note phrase a little later in the clip that is C5, C4, C5. If I start playback exactly on the first note of the phrase, all three notes are played as C5. If I start playback a little earlier, the correct notes are played. What is going on?

It's not the "Base octave for pitches" option because other notes are sounding correctly. Trillian has a base octave control too and adjusting that makes no difference, i.e. if I raise it an octave then the same problem occurs only an octave higher. 
 
I've tried various values for the MIDI buffer and also tried whacking my audio buffer up in various increments, but no joy. 
 
In fact trying playback from different positions in the clip creates different kind of note errors. For instance, starting from one particular note results in Sonar playing the note previous to it first (and then it screws up the octave of the note after the next note). 
 
Hmm, Sonar appears to have a problem with Trilian in a way I've never encountered before. I've tried outputting the clip in question to other instruments and it plays fine. 

Have you looked at the MIDI data in the event list?

Try it with a different synth & see what happens.
 
Load up a bass patch in Dim Pro, copy the Midi from your Trillian track to the DP Midi track.

I'm thinking that it may be Trillian by now.
 
Long shot, but is it a S.I.T. or a synth track + MIDI track?  If the former, is it worth splitting it?  It can't hurt to try.
 
I'm sure I've had a similar issue to this before, but can't for the life of me remember the outcome.

Well thanks for the suggestions everyone. The good news is that the problem seems to have magically disappeared tonight, although I sure as hell didn't (consciously) do anything to fix it. Well I never....this had me stumped yesterday, and it seems all it needed was a good old dose of 24 hours 

bitflipper
Coincidentally, I just experienced the same thing last night: one note intermittently playing an octave higher than written. It wasn't Trillian, though; it was a Kontakt library. It was particularly frustrating because I wanted to freeze the track but had no way of knowing if the note would be interpreted correctly until I'd played back the frozen track. I knew there was nothing in the MIDI data except note and pitchwheel events, because I'd hand-planted them via the PRV. This was not X2, but 8.5. I tried it again this morning, and the part played correctly. The computer had not been rebooted, nor had any processes been stopped or started, other than SONAR itself. Only the time of day had changed. My theory at this point: sunspots. 

I'm actually beginning to think that Sonar has a "time of the month."

Oh great. Now, the damn thing has started playing the first few notes of a clip a semitone higher. I'm clearly going to have to rework this tune into free form jazz. 

With soft synths, the problem for me has almost always been other tracks on the same MIDI channel introducing rogue controller or pitch wheel messages.  I always turn off all inputs on a track after I am through creating it.  I also always have a Controller 121 = 0 and a pitch wheel = 0 at the start of a MIDI track.  Finally, I always channelize when I am done recording: use Interpolate to set all msgs on a track to the same channel so I don't affect other tracks and so all events are recgonized.  (This can get messed up when recording parts from keyboards not necessarily transmitting the right channel.) 
 
With external devices (not your situation), my problems have usually been sysex (same device numbers).  If you ever load a MIDI file from someone else, you can get some bizarre sysex stuff.