Lead Conversion > Unexpected error when clicking Convert button using a Custom role

Question: 
 
Lead conversion > Encountering unexpected error when a custom role converts Lead:
Error 
  
An unexpected error has occurred. Please click here to notify support and provide your contact information.
 
The error does not clearly state that the problem is permissions-related.

Solution: 
1. Using Administrator role, navigate to Setup > Users/ Roles > Manage roles.
2. Edit the custom role used when converting Leads. 
3. The role already has Lead Conversion permission under Permissions tab > Lists
3. Check under Permissions tab > Transactions > add Opportunity (Level =Full) Note: Assign the level based on the need of the said role. 
4. Click Save.