  In this short note, we discuss the circumstances that can lead to a failure
to observe the design order of discretization error convergence in accuracy
verification when solving a time-dependent problem. In particular, we discuss
the problem of failing to observe the design order of spatial accuracy with an
extremely small time step. The same problem is encountered even if the time
step is reduced with grid refinement. These can cause a serious problem because
then one would wind up trying to find a coding error that does not exist. This
short note clarifies the mechanism causing this failure and provides a guide
for avoiding such pitfalls
