bad state transition?

I'm seeing the following trace:

|   srca   |Trying rule: [srca]                   |
|   srca   |Trying production: [srcb]             |
|   srca   |Trying subrule: [srcb]                |
|   srcb   |Trying rule: [srcb]                   |

Trouble is, srcb is not a production or subrule of srca.  

How do I even begin to debug this?

Meantime: I'll try to reproduce this error with a subset of my (large)  
grammar.

Thanks,
Jonathan.