I learned that Zip sometimes behave unexspected. From time to time we recieve Zip files which i am unable to open with the MacOS X 10.5.2 built in engine. The feeware "The Unarchiver" helps me because it proceeds regardless of an error. Maybe its Apples part to change the OS engine.
MacPro 12 Core 2,93 GHz, 32 GB RAM, Radeon 5870, 6 TB HD, XPress 9.2, CS5, LR 3