Sometime you get this error when you deployed a web part which use references. To fix the problem, you need to open your aspx page and add references like , even you are not useing it in your UI

A great post show you how to Packaging a WCF Service as a SharePoint Solution
http://www.intranoggin.com/Lists/Posts/Post.aspx?ID=23

This is the list of steps to follow for troubleshooting general SharePoint issues.

1. Turn on the debug flag from SharePoint web.config

Note: Enabling debugging is not recommended on a production environment.

a. Locate the SharePoint web.config file which is typically located under one of the virtual directories in C:\Inetpub\wwwroot\wss\VirtualDirectories.
b. Make a backup of the web.config just in case.
c. Open the SharePoint web.config using a Visual Studio or a text editor such as notepad.
d. Find “CallStack”
e. Change the value of CallStack to “true”
f. Find “CustomErrors”
g. Change the value of CustomErrors mode to ”off”
h. Save the web.config.

2. Reproduce an issue you’re having i.e. by refreshing the broken web page

3. Examine the error from EventLog. In most cases, you will find no useful information from EventLog since SharePoint does a poor job leaving a trace on the EventLog.

4. Go to SharePoint logs folder (c:\Program Files\Common Files\Microsoft Shared\web server extensions\12\Logs). SharePoint dumps all logs in this folder, and so you should be able to find clues by examining logs in this folder.

a. Sort items by Date Modified
b. Open the latest log file using a notepad or other text editors.
c. Scroll down to the bottom
d. Examine the latest log

5. Analyze error messages found from Step 2, 3 and 4.

6. Investigate issue for 10 minutes

a. Examine the system account used for an application pool of a SharePoint application. Does the account have proper permissions?

7. If you’re stuck for more than 10 minutes, stop what you’re doing.

8. Search for the clues from Internet — Many times, we forget how useful Internet is and that somebody else typically had the same problem before.

9. If no clues have been found, ask for help.

a. Describe detailed steps how to reproduce the issues
b. Ask colleagues or reach out communities such as the MSDN SharePoint Forums [1]

10. Iterate the step 1 to step 5 until an issue is resolved.

[1] – http://forums.microsoft.com/MSDN/default.aspx?ForumGroupID=328&SiteID=1

Just wanted to share a tip I find really useful. The idea being, I find myself going to the 12 hive quite often via command prompt.

Just create a file called 12.cmd and put it in your C:\Windows directory in the file put:
c:
cd c:\program files\…12… then from anywhere, just type 12 and voila!

First, create a Class Library project in vs 2005/2008.
Add Microsoft.SharePoint dll reference

Create a file called HelloWorld.aspx

<%@ Assembly Name="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c"%>
<%@ Page Language="C#" MasterPageFile="~/_layouts/application.master"
         Inherits="Microsoft.SharePoint.WebControls.LayoutsPageBase"  %>

<%@ Import Namespace="Microsoft.SharePoint" %>

<script runat="server">
  protected override void OnLoad(EventArgs e) {
    //SPWeb site = SPContext.Current.Web;
    SPWeb site = this.Web;
    lblSiteTitle.Text = site.Title;
    lblSiteID.Text = site.ID.ToString().ToUpper();
  }
</script>

<asp:Content ID="Main" contentplaceholderid="PlaceHolderMain" runat="server">
  <table border="1" cellpadding="4" cellspacing="0" style="font-size:12">
    <tr>
      <td>Site Title:</td>
      <td><asp:Label ID="lblSiteTitle" runat="server" /></td>
    </tr>
    <tr>
      <td>Site ID:</td>
      <td><asp:Label ID="lblSiteID" runat="server" /></td>
    </tr>
  </table>
</asp:Content>

<asp:Content ID="PageTitle" contentplaceholderid="PlaceHolderPageTitle" runat="server">
	Hello World
</asp:Content>

<asp:Content ID="PageTitleInTitleArea" runat="server"
             contentplaceholderid="PlaceHolderPageTitleInTitleArea" >
	The Quintessential 'Hello World' of Application Page
</asp:Content>

or you can create a sepeate cs file but add code into you

so now if you go to your site and add/_layouts/Test/HelloWorld.aspx, it should show the page