Complex root canal system is one of the reasons for root canal treatment failure. The more complicated the root canal system is, the more difficult to clean it thoroughly and higher chance to have a failure.
This case showed how a simple and straight root canal became complicated. Top front teeth normally have single and straight root canals and can be the easiest root canal treatments. What made this root canal system complicated were two accessory or lateral canals. They were impossible to be physically cleaned by endodontic files. But with thorough chemical irrigation, ultrasonic cleaning and firm root canal filling (obturation), they were cleaned and filled successfully. If they were left untreated, it's likely to have persist infection and eventually failed.