I am 100% sure it is a bug, you can even replicate it on your side if you also have a 2012 RTM server

just create a table like this

CREATE TABLE [dbo].[temptable2](

[Field1] [varchar](23) NOT NULL,

[field2] [varchar](20) NULL,

[field3] [varchar](50) NULL,

[field4] [datetime] NULL,

[field5] [varchar](255) NULL

) ON [PRIMARY]

Create clustered index idx1 on temptable2(field1)

load it with at least 100k rows

then do this:

select field3,count(1)

from temptable2

group by field3

it will never ever ends, now that is only happening on 2012, I just test it in a 2008 R2 sp2 server and it worked fine