Let�s take an out of the box deployment.
Go to Lead and create a new lookup field as account type, so a new relationship many-to-one between Lead and Account.
Add new account lookup field to �Qualify� stage in the Lead form.
Now, if you try to fill other attributes like �Topic� or �Last Name�, values are not store or showed properly in the UI and it does not allow you to save the record.
Same happens with Opportunity.
I have tried with a User lookup field and it works fine.