There is a big difference between mistakenly eating someone else's lunch and being a thief.

It's conceivable that someone could have a visually identical container (brown paper bag) and a visually identical lunch (sandwich and apple), in which case yes - that would be a mistake.

I'd bet that that is the case for much less than 5% of the lunches misappropriated.

Calling deliberate theft a "mistake" is fundamentally dishonest - it pollutes the meaning of the word mistake.

If you're a white-collar worker who is making a decent paycheck, you should definitely be accountable for the action of taking clearly-labeled food that doesn't belong to you. We should be merciful and try to help them, but that person is also committing a small form of thievery.

The spice was the correction. The fact that they tried to push back instead of owing up shows it's uncorrectable.

Usually people that I've worked with that have this kind of habit also have other habits, like copying open source into commercial code with no regard for license, checking in code under other people's names, flagrantly disturbing massive chunks of code (e.g. checking in whole-repository reformat) without telling anyone, etc.

It's definitely marginal and indicates a kind of thoughtlessness. Firing? Maybe not. But it's not easily forgotten.

Theft is a correctable habit? It's theft. The perp KNEW it wasn't his food, then when exposed, got HR involved to cover his tracks.

Please correct your habit of downplaying deliberate theft by calling it a mistake. Do you need any mentoring or coaching for doing that?