[racket-dev] TR tests sometimes fail with a promise error

Sometimes TR tests fail with the error

force: promise's thread terminated without result or exception

  promise: #<promise:!running!...et/optimizer/run.rkt:47:28>

  context...:

   /opt/plt/builds/<current-rev>/trunk/collects/racket/promise.rkt:98:2

   /opt/plt/builds/<current-rev>/trunk/collects/tests/typed-racket/optimizer/run.rkt:50:3:
for-loop

   /opt/plt/builds/<current-rev>/trunk/collects/tests/typed-racket/optimizer/run.rkt:44:0:
mk-suite

How can this happen? Is the thread getting killed? Also since this
error doesn't have blame information I don't know if this is an
internal promise error or if TR is misusing the promise library.