I just got into 3D Printing (got my Ender 3 two weeks ago) and I've been loving the experience! Learning lots of new things as well as making a few mistakes and learning from those.
I found this model and wanted to print it for a fish tank:
In some of the Make screenshots it looks like the model doesn't have a bottom. However when I brought the model into Cura, it looks like the model had a top and bottom portion.
Because of that, I thought it would need supports so I went ahead and printed accordingly. When the print finished, it was just like the Make screenshots provided. No big deal, it was super easy to remove the supports. However, reviewing the model in Cura I'm not sure how I was supposed to know that it wouldn't have the bottom printed.
In the photos it also looks like there's a low wall below the corner "window", which doesn't look like it's present in the preview. Perhaps the creator made different iterations of the model, and either used photos from an earlier prototype, or they uploaded the wrong model file.
I checked the linked model, and it has this almost break-away part in the middle (height-wise) as if the print is intentionally two halves you can can print at once or so. But then the window also looks completely different than in the pictures of the printed piece.
Man I have no idea! From the previous comment I thought it made sense. I redid the print yesterday and it printed exactly how it looks in Cura with a top and bottom.
In cura switch to the preview mode to see the paths your printers will lay plastic at. It will tell you a better idea of what your final product will look like.
That is showing the model, but you can press the print preview button and it should show you how it will print. If that was just a superfluous polygon that wouldn't print, it'll show up in the print preview.