Accidental complexity is complexity that arises in computer programs or their development process which is non-essential to the problem to be solved. While essential complexity is inherent and unavoidable, accidental complexity is caused by the approach chosen to solve the problem.[1]
While sometimes accidental complexity can be due to mistakes such as ineffective planning, or low priority placed on a project, some accidental complexity always occurs as the side effect of solving any problem. For example, the complexity caused by out of memory errors is an accidental complexity to most programs that occurs because one decided to use a computer to solve the problem.
While accidental complexity is to be minimized in any good architecture, design, and implementation, excessive accidental complexity is an example of an anti-pattern.