Hi, I own a mid 2012 rMBP running Mountain Lion. I had this issue before and got it fixed by formatting and reinstalling ML (did not do that in order to fix it but it came with the package). Anyway, I didn't have this until yesterday, when updating to 10.8.4, it happened again. The issue is.. everytime I try updating something from the MAS, it gives me an error even though it does update. Example: With OS X updates it downloads, logs out, the MAS window with the progress bar appears and, when it's reaching the end, an error appears (don't know what code or if it gives any code), it restarts and, when I come back and check, the update disappeared and I already have the new OS X version. Today was with iTunes 11.0.4. Updated, when the bar was reaching the end, error 103 but, again, the update disappeared and I had iTunes 11.0.4 installed. What the hell is going on? Anyone else with this issue? It's really annoying and I'm afraid that it's doing something under the hood that's not good for the OS. I mean, it must come from somewhere. Any idea, please?