Despite the issue was addressed in b3d6cda, I started to get this this error time-to-time in my dexbot production instance.

Error means "referenced block num prefix != actual block prefix". Block prefix is taken from next block using "previous" field: block = ws.get_block_header(int(dynBCParams["last_irreversible_block_num"])+1)

Some ambiguity is caused by referencing block num using only 16 bits. This means, referenced block num should not be older than 65535 from head block. The only way I can reproduce this issue artificially is by referring to some old block, like this: dynBCParams["last_irreversible_block_num"] -= 70000

I'm not sure how it's happening in production, but I can guess there could be a reconnection to the next node and LIB on new node was shifted to the past (node out of sync). I added a debug to see LIB next time the issue will happen.