You shouldn't ever get these errors, even if you forget performGC. I've heard from Michael Xavier that he had this issue when he switched to ghc-7.8. Are you by chance also using ghc-7.8?
Either way this is a bug since it violates the library's contract. If you can throw together a minimal example that reproduces the error as a Github issue then I can dig into this.
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I'm pretty sure `pipes-concurrent` is the problem because it is doing unsafe stuff under the hood (using `unsafeIOToSTM` to touch a weak `IORef`).
Fortunately, there is a safe alternative now, which is the newly added `mkWeakTVar` in the `stm` package.
I will fix this on the weekend but if somebody has time before then the above solution is the first thing I would try.