From Minecraft Wiki
Beta 1.5_02 was a server-only release to fix a few issues with Beta 1.5_01. (Beta 1.5_01 clients would connect to servers running Beta 1.5_02).
2 bugs fixed
- Game no longer crashes from "internal server error" created by bugs in 1.5_01's Server in SMP
- Players no longer get the Saving Chunks screen when taking something out of the Crafting Table