When the devil in the details appears something is seriously wrong with the design. Usually, you will see symptoms of bad designs in the code.

The following are the design smells that Robert C. Martin mentioned in his book, Agile Software Development: Principles, Patterns, and Practices:

• Rigidity: The system is hard to change because every change forces many other changes to take place in other parts of the system
• Fragility: Changes cause the system to break in places that have no conceptual relationship to the part that was changed
• Immobility: It is hard to disentangle the system into components that can be reused in another system
• Viscosity: Doing things right is harder than doing things wrong
• Needless complexity: The design contains infrastructure that adds no direct benefit
• Needless repetition: The design contains repeating structures that could be unified under a single abstraction
• Opacity: It is hard to read and understand. It does not express its intent well

Among these design smells, rigidity, fragility, immobility, viscosity, and opacity are the symptoms of bad designs. Needless to say, complexity and needless repetition are two of the causes of those symptoms. Let's look at all of these symptoms closer.