In the SalesLogix web client, for all 8.0 versions up to core update 03, if there is a lead that contains activities, the lead cannot be deleted or converted to a contact/account successfully unless all of the activities are for the SalesLogix user attempting the delete. This is also true for the admin user- all activities must be for admin to do the delete.
When attempting to delete a lead you get an unhelpful error exception, requiring you to use the browsers, back button.
When attempting to convert a lead it is a little worse. The logic of the lead conversion is that the first step it takes is to create the account/contact records and then to delete the lead data. The problem is the delete fails due to this issue. The end result is a new contact/account is created but the lead remains as well.
I was told that core update 04 will address this issue.