Multiple optimal solutions
A common misconception among beginners in optimization is the idea that optimization problems really have just one solution. Surprisingly, this is typically not true. For many practical problems, the objective (whether it is cost or revenue or ...) is dominated by a handful of variables, while most variables are just there to ensure that the actual operation of the solution is possible. Consider a staffing problem, for example, where cost is typically driven by the number of people who work on a given day, not by the specific people.
These kind of situations naturally lead to problems similar to
Graphically this can be depicted as