Apple has reportedly discovered a critical flaw in watchOS 2 and decided to withhold the update to fix it, pushing back today’s scheduled release. The issue may remind users of a similar predicament with HealthKit applications at the launch of iOS 8 last year.

Users running the GM build of the software haven’t yet reported anything big enough to potentially stop the entire release, although there were some reports of sluggish performance throughout the beta. Many users have noted that iOS 9 suffers from the same performance issues on some devices, even newer iPhone hardware, so it’s unlikely that was the cause for this change of plans.

A source with knowledge of the issue told us this problem won’t impact the release of iOS 9 later today.