I'm experiencing a similar bug in the full version, even after having applied the patch linked to on the main page. The first time I save, the game saves successfully, and the music continues, but the graphics lock up entirely and the game becomes completely unresponsive. After this, I must force quit the application. However, when I start it again, I can load the save file successfully (although the screenshot for the save file is a blank white image). I am playing with a gamepad, at the highest supported resolution, in windowed mode. I immediately attempted to save the game again, and the exact same crash occurred.
I'll attempt it later without a gamepad, in fullscreen mode, and/or at lower resolutions, but for now I'm just enjoying it too much to stop.
Update:
I've tested it with a good combination of settings and it is happening in all of them. I started a session of play (640x480, windowed, gamepad enabled, vsync enabled, framebuffer effects enabled, mipmap enabled) in which I immediately saved, and this is the part at the very end of the debug.log file that seemed relevant to the saving process.
pos [-1], file :gfx/save-point-glow
LOADED TEXTURE FROM DISK: save-point-glow ref: 1 idx: 737
pos [18], file :./save/screen-0000.tga
Unable to load TGA File!
LOADED TEXTURE FROM DISK: ./save/screen-0000 ref: 1 idx: 738
pos [18], file :./save/screen-0001.tga
Unable to load TGA File!
LOADED TEXTURE FROM DISK: ./save/screen-0001 ref: 1 idx: 739
pos [18], file :./save/screen-0002.tga
Unable to load TGA File!
LOADED TEXTURE FROM DISK: ./save/screen-0002 ref: 1 idx: 740
pos [18], file :./save/screen-0003.tga
Unable to load TGA File!
LOADED TEXTURE FROM DISK: ./save/screen-0003 ref: 1 idx: 741
save sized
saveSizedScreenshot
push
disable blend
wbit: 512 hbit: 384
zx: 0.8 zy: 0.8
w: 512 h: 512
width: 640 height: 480
copyw: 640 copyh: 640
pixel zoom
copy pixels
It cuts off just like that. I'm going to try updating my GPU driver next (although it's never given me any problems before).
Additional update: Updating to the newest version of my GPU drivers cleared the problem right up. No more such problems on my end - though I do hope, for the sake of people who might otherwise miss out on this game, that future versions correct this so that it fails gracefully on less up-to-date systems.