Don't use different WF versions for SPD and MOSS 2007/WSS v3
Well, the title just says so... 
My MOSS 2007 server was running on a PC with Windows Workflow Foundation (WF) Beta 2 installed. My workstation has a Microsoft Office SharePoint Designer (SPD) 2007 Beta 2 TR with .NET Framework 3.0 RC1. The result? Aaarrrgghhhh... the custom workflow I created on SPD couldn't run on my MOSS! An error message appeared, saying that the workflow had errors, it was saved but wouldn't run.
Cak Riwut suggests me to try downgrading the WF on my PC. So I just uninstalled .NET Fx 3.0 and installed WF Beta 2 after that. When I reopened the custom workflow on SPD, it was a mess and couldn't be edited... so I deleted it and recreated the same thing from scratch. Guess what, it ran smoothly. Thanks Cak!