Apple squashed scores of security flaws in updates to OS X (10.11.5), iOS (9.3.2), and watchOS (2.2.1) on Monday, and TMO recommends that you run those updates ASAP—unless you're on an iPad Pro (9.7-inch).
In fact, let's tackle that first. MacRumors first noted that a subset of users were reporting getting "Error 56" when running Apple's Over-the-Air update (OTA) on their iPad Pro (9.7-inch). Those who encountered that error were sent to a loop where they were instructed to connect to iTunes, but doing so didn't help. In short, their devices were bricked, hopefully temporarily.
Accordingly, your humble writer bravely didn't run the OTA update and instead updated through iTunes. That update went swimmingly, but we'll never know if my device would have been bricked otherwise. If you're squeamish, hold off a day or two while this shakes out. If you're not, update through iTunes (which I usually recommend anyway). If you're brave and full of piss and vinegar, throw caution to the wind and risk bricking your device.
For everyone else, get thee to the updates!
OS X 10.11.5 includes three bug fixes and 45 security patches. All of those security flaws are serious, too.
iOS 9.3.2 includes five bug fixes and 22 security patches. Again, all of them are various degrees of serious.
watchOS includes unspecified bug fixes and 15 security patches.
While we're at it, Safari 9.1.1 includes 4 security patches, all serious.
iTunes 12.4 includes a half-assed return of the side bar (more on that after we've had an evening to play with it). We don't yet know if there were any security patches addressed in the release on OS X, but the Windows release notes specify one security patch.
This is a serious barrage of security patches, and many of them allow the bad guys to take over your device or steal information. As such, run the updates as soon as you're comfortable doing so.