Same problem here:
Client> java.lang.StringIndexOutOfBoundsException: String index out of range: -1
Client> at java.lang.String.substring(String.java:1937)
Client> at asm.a(SourceFile:1469)
Client> at asm.a(SourceFile:1461)
Client> at asm.k(SourceFile:1452)
Client> at asm.K(SourceFile:300)
Client> at asm.c(SourceFile:503)
Client> at net.minecraft.client.main.Main.main(SourceFile:76)
Client> b@388aaff4
Client> 2013-04-19 09:00:21.816 java[56611:707] Unknown modifier with keycode: 0
Client> 2013-04-19 09:00:25.862 java[56611:707] Unknown modifier with keycode: 0
Client> 2013-04-19 09:00:41.958 java[56611:707] Unknown modifier with keycode: 0
Client> 2013-04-19 09:00:59.861 java[56611:707] Unknown modifier with keycode: 0
Client> 2013-04-19 09:02:05.317 java[56611:707] Unknown modifier with keycode: 0
Client> 2013-04-19 09:02:54.605 java[56611:707] Unknown modifier with keycode: 0
If I just let the game sit at the mojang screen, the "Unknown modifier with keycode: 0" just keep coming up.
This is what comes out of mine:
Client> java.lang.StringIndexOutOfBoundsException: String index out of range: -1
Client> at java.lang.String.substring(String.java:1937)
Client> at asm.a(SourceFile:1469)
Client> at asm.a(SourceFile:1461)
Client> at asm.k(SourceFile:1452)
Client> at asm.K(SourceFile:300)
Client> at asm.c(SourceFile:503)
Client> at net.minecraft.client.main.Main.main(SourceFile:76)
Client> b@228ca8ac
Client> 2013-04-18 17:10:13.751 java[54870:707] Unknown modifier with keycode: 0
Client> 2013-04-18 17:10:37.771 java[54870:707] Unknown modifier with keycode: 0
Client> 2013-04-18 17:11:53.238 java[54870:707] Unknown modifier with keycode: 0
The three "unknown modifier with keycode: 0" appear after I let the Mojang screen sit for a minute or two.
Still having same problem, followed Grum's instructions but nothing's changed. I tried re-insatlling java, deleting my current minecraft folder and redownloading the launcher, but nothing's worked.