What? I know how to open a file! Jeez!
OK. Just in case:
Always open AC files via right click -> Open With or by dragging the file to requisite AC icon on the dock. I do the dock thing; I'm just not a context menu person usually.
Most important, don't double-click PLNs. Double-clicking will only give consistent, predictable results if you have exactly one copy of AC on your machine.
When you install a new version of AC, as you soon will in the form of 11, that version becomes the default application for opening PLNs. So you double-click your AC10 project, it opens in 11, you don't notice the difference because there isn't much, you save it, and next time you try to open it in 10 it doesn't work. (Yes, backsave. Real convenient.)
Always keep current versions of AC on the dock. Even if you use the right-click method, the dock gives you visual confirmation that the right application is being used.
Graphisoft gives poor support in this area, by making the icons identical and naming each version, precisely, "Archicad".
Double-clicking files is one of those 'automatic' things that's great if you can trust it, but when it's not predictable it makes trouble. It's not just AC either, recently my .docs started opening in Pages for some reason.
What about that 'Change All' under Open With in the Get Info window? That fixed my .doc issue, but it doesn't reliably assign the right 'Archicad' version.
So, right-click or dock. If you have a bad habit here, change it now before 11 starts getting on your nerves.
Update re app icons: Mr. Briggs to the rescue.
