When you're logged in to the same iCloud account on multiple Apple devices, your open Safari tabs are kept in sync. That goes for both iOS and OS X devices alike.
A handy feature, no doubt. But there are times when it can lead to mass panic and hysteria. Let's say you share a computer or iPad with your significant other, and you forgot to close a tab you really would rather they not see.
Did you know you can remotely close open tabs on another iCloud-connected device? I didn't, then I accidentally found out over the weekend and had to share it with you.
The next time you need to close a tab on another device, don't panic. Launch Safari on your closest Apple device, then switch to tabs view. On iOS, tap on the icon in the lower-right corner. (OS X users will have to wait until Yosemite is released for instructions, though I'm guessing it'll be near-identical to iOS.)
Scroll down to the bottom of the open tabs page, where you'll find your iCloud devices listed by name, and the tabs open on each.
Find the tab (or tabs) you want to close, and swipe to the left across it. A red delete button will appear, just as it does in Messages or Mail. Selecting delete will close the tab within a matter of seconds on the device, even if it's currently being viewed.
Looking back at how this feature works, it's painfully obvious and I'm not sure how I missed.