Don't blink. Blink and you're dead. Or more likely, sent back in time if you happen to be touched by an angel (HAR HAR HAR). The Weeping Angel is one of the deadliest creatures in the Whoniverse, able to send people back in time, or break necks. Speaking of breakneck, that's the speed they can move at when you're not looking at it.
SPOILERS: A few-
River Song: "Spoilers" is my line, sweetie.
O-kay... so, in the pyramid, after losing to Bard in Season 2, a Weeping Angel returned in Season 3 to defeat Geolix, Scooby-Doo, and Hawkeye, and is currently waiting for competition in Round 3. Blink and you'll miss it.
Season 2: TheAnonymousA
Season 3: Weasel23