Why would you be deported?

No passport - EVERYONE must have a passport to travel into and out of a country. Although it doesn't always happen, EVERYONE trying to enter Another Country without a valid passport SHOULD be refused entry to the country they're trying to enter.

No visa - Even if you have a passport, some countries require you to have an entry visa.

Undeclared convictions - Many countries will refuse someone who has criminal convictions (dependent on the seriousness of the offence)

No means of supporting yourself - If you don't have access to sufficient funds during your stay, you may be forced to beg (which is illegal in most countries)

No-where to stay - Again, having no accommodation will force you to sleep rough, which many countries don't allow

Wanted criminal - If you're a wanted criminal in the country you just left, you should be returned there to face whatever charges are levelled against you.

In ALL the above scenariOS, you don't have an automatic right to enter the country of destination, and (quite rightly) should be returned to your country of origin !