Since it is impossible to test many large programs exhaustively, bugs can slip through the debug stage, only to show up months and even years later. Fixing such bugs is an important maintenance task.

• 由于不可能彻底地测试许多大程序。在调试阶段中，一些缺欠可能滑过去了，几个月甚至几年以后才暴露出来，测定出这些缺欠是非常重要的维护任务。