Thumbnails in Aperture are Really Messed Up

Having a ton of trouble with both Aperture and iPhoto. The thumbnails in both products are showing up in a random mix/match of landscape and portrait shots, and random order. I've read the suggestions on this site, and have tried using both the Photos > Generate Thumbnails and Photos > Generate Preview features. Both temporarily fix the Thumbnails in the current view (ie the ones I can see), but as soon as I start to scroll, its back to a mess again.

It seems the next step is to rebuild the database ... but what I don't understand is WHY Aperture would ever be in this condition in the first place? Rebuilding any database is not a small step to take. And its not clear this would even solve the problem. I mean, Aperture is worthless without a reliable thumbnail, so how is it possible for the product to show a thumbnail for one image when the actual image is something else. This is about as basic as it gets!

In other words, it feels like a really heinous bug in the product, that is likely to resurface if I continue doing whatever I did to get here in the first place.

Anybody have some suggestions/explanations on why it would behave this way, and how to *permanently* fix the problem? Thanks in advance!