A common source of numerical issues is numerical rounding in the numbers that are used to represent constraint matrix coefficients. To illustrate the issue, consider the following example:
So, with this in mind, during presolve Gurobi can use the second constraint to determine:
If user had provided these two equations instead:
The main point is that constraints that are exactly parallel, or linearly dependent (within double-precision floating-point and small tolerances) are harmless, but constraints that are almost parallel to each other produce tiny coefficients in the linear system solves and in preprocessing, which can wreak havoc on the solution process. In the next section, we expand on the limits double-precision floating-point numbers, and in particular why .