I prematurely marked an exercise as complete - it passed tests on my system but wound up failing on the exercism runners. (This may be a boost version issue that needs further investigation.) I’d like to return the exercise to “in progress” while we iron this out.
Is it possible to revert the “mark as complete” decision?
Deleting all iterations should return the exercise to “in progress”.
Thanks, but I have an active mentoring session in the iteration that I want to keep. Any other way?
Maybe delete all iterations and then re submit the iteration you want to keep? It should show up in the mentoring session automatically.
An argument could be made that this is simply a missing feature.
Moving to feature requests 
No, there’s not a way. Completing exercises comes with consequences (badge awarding, emails, unlocking of other things, etc), so it’s not purely a database state. So we just don’t allow it - sorry.
On the other hand, “completed” does not mean “fixed in stone”. You can submit more iterations if you want.
Also note, completed doesn’t prevent you from continuing to comment on a mentoring session or opening a new one.