OK. Those materials were still being used in your model. I removed all of the materials from that one. Lots of reversed faces in this version. There should be no blue back faces.
Still an issue with joint fit.
Stackable Chair no mats.skp (239.9 KB)
If you are planning to use this model as a plan to use in your shop so you can build it, it would be wise to increase Precision in Model Info. Also turn off Length Snapping as that tends to get in the way of precise modeling.
You can see the result of increasing Display Precision in your model.
Those errors in your model might not be a problem but then again, they might. With the coarse precision you have set in the model, you don’t have an opportunity to find out before wood is cut.