It seems that when search engine bots crawl my websites, I get errors in the Event Log related to the Event module. Based on the error log (sample below), it seems that the Events module has trouble when the "user" is a search bot. This happens to me on several portals (different Events modules) and different bots including Googlebot and msnbot. So, perhaps the Events module is assuming something about the user context or browser capabilities that bots dont't have.
I've had this problem for awhile, but I am now running DNN 4.9.0 and Events 4.0.2 and it still occurs.

Typical Event Log error:

ModuleId: 2212
ModuleDefId: 95
FriendlyName: PrintableEvents
ModuleControlSource: DesktopModules/Events/Events.ascx
AssemblyVersion: 04.09.00
PortalID: 5
PortalName: ********
UserID: -1
UserName:
ActiveTabID: 1435
ActiveTabName: PrintableCalendar
RawURL: /PrintableCalendar/tabid/1435/ModuleID/2212/ItemID/1097/mctl/EventDetails/Default.aspx?selecteddate=6/1/2008
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer:
UserAgent: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: f72ab608-d34b-437c-bf36-5902d4558c06
InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.Modules.Events.EventDetails.Page_Load
StackTrace:
Message: DotNetNuke.Services.Exceptions.ModuleLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Modules.Events.EventDetails.Page_Load(Object sender, EventArgs e) --- End of inner exception stack trace ---
Source:
Server Name: *****

I changed the name of the portal and Server Name.