Reliability is remembering that another person may be building around your promise.

When you promise something, someone starts building around it. They arrange their time, tell someone else, or stop pursuing an alternative. Breaking the promise does not just cost them the thing, it costs them whatever they gave up in reliance on it, which is usually more.

Before you let something slide, ask what has been built on top of it. If you must break it, tell them early, while their arrangements can still be changed.