Some issues...

With Python

• slices are copies
• dict iterate on keys
• triple inequalities (False == False in [False])
• rely on exception types but ignore values (__getattr__ raising AttributeError is silenced)

With OOP

• hard to figure which concrete method will be executed

In Existing Codebase

• naming
• separation of concern
• premature optimization
• magic values
• partial migrations
  • renaming half done
  • database not cleaned up
• implicit dependancies
  • using static references
  • relying on other component data
• spagetti code
  • calling unrelated modules