Debugging sharepoint error

Scenario:
You was working with SharePoint and got Error : Unknown Error or Cannot complete this action

Reason:
SharePoint has encountered some processing error but haven't displayed the complete Error details because of various setting in Web.Config ( relates to Debugging and Trace etc ).

Resolution:
0. Check the logs in 12 HIVE
1. Make the necessary changes as given below to the Web.Config.
2. Make sure Logging is enabled in Central Administration > Operations > Logging
3. Check the logs in 12 HIVE

Code:

<configuration>
<SharePoint>

<SafeMode CallStack="true" AllowPageLevelTrace ="true">
** change both to true

</SharePoint>
<system.web>
<customErrors mode="RemoteOnly" />
** change to RemoteOnly (better than Off)

<compilation debug="true" />
** change to true

<trust level="Full" originUrl="" />
** may need to change this to Full

</system.web>
</configuration>

Article
Custom Errors