I've never had any problems with OSX Software error messages before! Last night after starting the update to 10.5.7 though I got this frightening assertion:
The update "Mac OS X Update" can't be saved. The digital signature for this package is incorrect. The package may have been tampered with or corrupted since being signed by "Apple".
I'm betting on the corrupted part, but I think I'll read around some before I try it again today.