I had a Samsung Galaxy SCL i9003 began to malfunction; I just restarted, lost contacts, program settings, etc.
After searching the web, it turned out that the internal FLASH memory chip, using, being of low quality, began to fail.
Such memories theoretically not lose their data for up to 10 years, but if damaged by use (allow a limited number of cycles, about 100,000 or more), you may not withhold good data and "forget" . It can happen to any phone that uses them, and sooner or later any "smartphone", tablet, ebook, etc with internal flash now have this problem.
To check it, it is best to use this freeware program h2testw . We connect the phone in "mass storage" mode and run mode continuous writing and then read until an error appears, checking the box for "endless verify" (to select target drive select):