For more than a decade, Apple’s silicon engineering group has helped give Apple a competitive edge in smartphones and laptops. The custom chips it designs make its products snappier to use or help them eke out an hour or more of extra battery life. Last year, though, Apple’s chip department hit a serious setback.

Apple planned a generational leap for the graphics processor in the latest version of its high-end smartphones, the iPhone 14 Pro. But engineers were too ambitious with adding new features, and early prototypes drew more power than what the company had expected based on software simulations. That could have hurt battery life and made the device too hot, according to two people with direct knowledge of the incident. Because Apple discovered the mistake late in development, it had to base the graphics processor in its iPhone 14 Pro line—which powers the phone’s user interface, games and everything else visible on its screen—largely on the design of the chip that went into last year’s iPhone model, according to four people familiar with the matter.

Those people described the snafu, which hasn’t been previously reported, as unprecedented in the group’s history. The iPhone 14 Pro models, which went on sale in September, showed only small gains in graphics performance compared to the leaps prior generations of iPhones had made over their predecessors, according to testing by independent chip-benchmarking firms.