Showing snippet of previous employer's source code during interview?

I've had this happen multiple times while interviewing. The most memorable was a fellow who brought his previous employers entire codebase and didn't seem to understand when we questioned him about his former employers IP policies. Better yet he brought it on his 'personal' laptop which had a sticker from his former employer's IT department on it with barcode, machine id, etc.

Suffice it to say this is an instant no-hire just about anywhere even if you're not so blatant about it.

Instead here's the advice I've given everyone I've seen do this in an interview - build an application that scratches an itch for you or contribute to an open source project you're interested in. This is great for employers because it's crystal clear what code was written by you and an itch-scratcher might even earn you some semi-passive income.

Reminds me of a story which Tom Limoncelli tells in which a very good candidate for a sysadmin position came in and impressed everybody, but the diagrams of server solutions the candidate showed them were all marked as being confidential internal documents of the candidate’s previous employer. Because of this, they did not hire the candidate.