I found out some strange things on the grid, when I collected data on Titan krios.
How was it caused? Is that ice contamination? Is that caused by dirty liquid nitrogen? Or something happened in the scope? Even, another grid which another co-worker independently prepared are the same.
Most likey the grid was devitrified while being handled during preparation, clipping, or before loading into the microscope.
Thanks for your reply 
. I will try it again and be careful.
Good luck! It’s happened to everyone many times, I assure you. Wait for the nitrogen to still and cool down your tools off to the side before touching the grids/buttons. Keep in mind there is about a 3mm safe zone of cold, dry gas above the liquid.