Wow, that's a super-long-exposure photo of http://www.testufo.com/frameskipping
-- if I see correctly, you used a 1 second exposure? Almost every square is (barely) billuminated except for two squares near the bottom-left corner. No skipped squares, though, so 66Hz looks good. For easier overclock benchmarking, you do want to shorten camera exposure (but not too much) it so that several squares shows up (up to around 1 or 2 rows worth) -- you only need to verify contiguousness of squares, not highlight every single square in the TestUFO Frame Skipping test!
Next time, just set your camera exposure to approximately 1/10th second.
Either way, 66Hz overclocking *seems* to be working.
Going above an overclock limit will cause visual glitches. When you see this happen, stop what you're doing, back down a bit. If you get heat issues, you'll probably see instability occur. If this happens, stop using the overclock and back down. Also, you can get instability issues with no heating/damage too, but you never know which electronics react with prolonged use of an unstable image. The rule of thumb; stable image is generally safe.
If you see any side effects such as temporary burn-in occuring (stuck ghost images that linger for minutes), then stop overclocking, play a video (or fast-changing imagery) to erase the temporary burn-in effect -- this only occurs on monitors when LCD inversion circuits fails during overclocking (rare), or incorrect overdrive overvoltages occurs (very very rare), etc. All rare, but worth paying attention to.
Stable image will generally be safe on the vast majority of displays.
It's very rare for overclocking (stable image) to damage any display, especially if you're only doing such a small overclock (60Hz->66Hz) -- unlike this user who did a successful 60Hz->180Hz overclock
(Very rare to get such a huge overclock!) which is far more risky.