Factorio Forums

An exit-only underground belt is a simple, quick, compact and cheap method of getting just only lane off of a belt. And it works great! Unless it happens to be facing an existing unmatched underground belt when you place the blueprint. If that happens, it changes to an input instead of an output, and the blueprint suddenly doesn't work correctly.

To reproduce:
1. Create a blueprint with a single outputing underground belt.
2. Place the blueprint somewhere.
3. Rotate the blueprint and place it again, in range of and opposite the first one.
4. Observe that it's now an input underground belt instead of the output that the blueprint calls for.

I do not believe that it matters if the belt from step 2 is still a ghost in step 3, but I haven't tested that.

I did (manually) create a blueprint with opposing underground exit belts, and it was also modified, and could not be created correctly in-game. This was what actually led to this report, as I have several blueprints that use an outputing unmatched underground belt, but haven't actually encountered the effect of this bug with them. But attempting to create a maximized belt to chest to train loader using unmatched output underground belts to attain the maximum possible stack inserter belt->chest throughput proved to be impossible.

This is intentional.

I'm not surprised, and it's what expected and need, when manually placing belts. With blueprints, It has the unintentional side-effect of making certain blueprints sometimes not work. If a blueprint says to put in something a certain way, I would think that's how it should be put in. Certainly changing away from what the blueprint says is unlikely to result in a functional parts flow!

Basically, this behavior makes sense when manually placing belts. It does NOT make sense when doing so with a blueprint, as it results in unpredictable malfunctions. Not horribly often, admittedly, but still results in malfunctions with no obvious cause.

this is a bug for the following reason:

Blueprints don't always create what's shown in the blueprint.