Repeat after me, "My USB flash drive is going to fail". It's going to be in your pocket and go through the washing machine. It's going to get dropped on the concrete and bust into pieces. It's going to randomly lose files after you shock the door handle from static. It's going to get kicked out of the front of the computer and bent. One way or another, it's going to fail and you should never keep the only copy of important files on any USB flash drive (or external hard drive, but that's another discussion).
This picture features the guts of an old 512MB USB Flash drive a customer brought me this morning. It has been used heavily and was always carried in a leather sheath. Today it stopped working for her. Typically when I open them and find physical damage, it is limited to the wires on the back of the USB plug. We fix those, and about 50% of the time, we can see data again. This time, I found this. The memory chip itself was detatched from one side. This whole part isn't much larger than my pinky's fingernail. I did not have the tools for a proper repair attempt, but I still managed to engineer it and get hundreds of patient files off of this drive. Even I was amazed. However, the lesson still stands. Backups of important files are necessary because you will not be as lucky as this customer was.