HOW CAN I OVERCOME THIS PROBLEM APPEARING IN MY mw3
By using the "Search" button, and not posting in CAPS.
Here's some spoon-feeding:
"The solution is not to go along the street but jump inside the bulding with broken windows on the right. There will be several enemies there, you will kill them, exit the building on the other side and the checkpoint will save. Going along the street in the open causes this glitch."