I’m sure you all already know this, but it caught me out today and I should have known better. Also I realised I haven’t blogged for a while, mainly because I am working on application page for MOSS containing mainly standard .NET code that is not really SharePoint related.
When you make some changes to a DLL in the GAC, e.g.a workflow or an application page. You must do an IISRESET after you copy the new/replacement dll into the Global Assembly Cache. Failure to do this will mean the code continues to use the old dll.