[plt-dev] Mysterious "link: module mismatch" bug

Running code in DrScheme from one of my planet development links, I
get this familiar error:

link: module mismatch, probably from old bytecode whose dependencies
have changed: variable not provided (directly or indirectly and at the
expected position) from module:
"/Users/cce/research/planet/scheme/syntax.ss" accessed from module:
"/Users/cce/research/planet/scheme/require-provide.ss" at source phase
level: 0 in: provide/contract-contract-id-make-planet-path.62

The usual process to fix this is to recompile the relevant files.  I
have now twice cleaned out all compiled files from my planet
development links and run a full 'setup-plt' to restore them, and the
message has not gone away.  What could be causing this, if the .zo
files are all up to date?

Carl Eastlund