C2 r59; Windows XP SP3; FireFox 7.0; ATI Radeon HD 4870
Dual-core E5200 2.5 GhZ; 3,5 Go RAM
It appears that in an existing project, when adding new animations and add frames/load textures to it and save as folder project in a new folder, the frames disappear from the project.
(At least under windows XP)
The error will produce a check failure when you try to launch a preview.
DON'T DO IT !!!
Also don't close the project or C2 or try to open it again right away, you'll get errors.
Instead, check the animations you just created.
It should appear like this:
<img src="http://dl.dropbox.com/u/36472942/construct/bugs/R59-missingsavedanim-small.jpg" border="0" />
Just load again your textures and set the origin to where you wanted it. (When I found the bug I didn't messed with the origin points, they were set to the center, where I wanted them).
Once you're done, save the project (simply save, not save as) and now you can close the project and open it again. From there you can launch previews and work more.
(If you had launched a preview after having saved right now, you would have had a JS assertion failure occuring in your browser.
I've sent a bug report to ash in MP (big project/assets) as well as a step by step to produce the bug. (apparently it occurs when saving as new folder)
Just be careful, this work around works for now.