Accidents caused by poor design, failure to follow listed safety procedures, or other forms of human error are not considered to be beyond-design-basis accidents. The terminology can be unclear, however, because a poorly handled design-basis accident can result in conditions beyond what was considered likely, causing a beyond-design-basis accident. For this reason, some industry experts have criticized the use of design-basis terminology. The Three Mile Island accident and the Chernobyl disaster are examples of design-basis accidents becoming non-design-basis accidents largely due to failures to follow the written operating procedures.