Apple is facing another potential issue with the iPhone 4, according to a new report. Citing sources both inside and outside Apple, Ryan Block of GDGT reports that the company has found that certain iPhone 4 cases—specifically those that slide-on to the device—are prone to small particles getting stuck between the case and the rear glass panel of the iPhone 4. While the unexpected scratching that can occur due to such particles is not a new phenomenon, on the iPhone 4 these small scratches have the potential to develop into larger cracks or even full-on fractures in the glass rear surface of the phone. The report states that Apple has kept all slide-on iPhone 4 cases off its real-world and virtual store shelves as a precaution, and has created a new lab and testing program to investigate the issue further. The repercussions for companies that spent time and money developing slide-on cases for the device remain unclear, as Apple has yet to say when—or if—it will allow such accessories back into its stores. In the meantime, many such cases continue to be sold online and through other retailers.