WinBUGS is renowned for its cryptic error messages. Recently, while trying to fit a logistic model, I have been getting the message ‘Trap 66 (postcondition violated)’. There is not much on the web on the possible causes for this. There are however a few mentions such as on page 10 of this document and on page 251 of this document. In both cases, variance priors are identified as the culprits. Indeed, in my case changing the prior on the random effects standard deviation from dunif(0, 100) to dunif(0, 10) solved my problem.
PS: This post seems to attract some traffic probably because it is a common problem with no clearly identified cause. It might help if people who have encountered this problem could leave a short description and how they solved it as a comment.