Decoupler failed to decouple

[LittleBitMore]

While landing Tim C on the Mun to go rescue Bill, I ended up with a bit more dV than planned, and didn't need to stage the transfer stage until landing. Upon running out of fuel, I attempted to detach it, but it did not detach. It seemed like it was internally another craft, since I could select and focus the spent stage separately from the lander, but it wasn't physically budging from the lander. Strangely, the landing engine was able to fire through the spent stage, and had enough TWR and fuel to soft land (but it toppled and the mission failed).

The decoupler was between a Poodle engine and a jumbo 2.5m tank, if that helps any.

[schlosrat]

I've seen this happen a number of times. The workaround I've found is to NOT have the decoupler and the next engine in the same stage. It's annoying, but if you give the decoupler a little time then the spent stage clears the next stage and you can fire your engine. I've not seen this kludgy method fail yet, but I've seen several instances where, if the engine and decoupler are in the same stage group, then I end up towing a spent stage behind me with the engine firing.