Random contacts on my iPhone 5

Solution 1:

This is likely because iOS 6 has Facebook and Twitter contact import. So, if you logged into your Facebook account on your iPhone, it will automatically go through your contacts and pull in information that it finds in their Facebook account.

Solution 2:

It may be a longshot, but in addition to the Twitter/Facebook contact import mentioned above, are you perchance using Gmail, and syncing iOS to your Google Contacts via CardDAV?

If so, you're probably seeing Google's 'My Contacts' in your address book. For every person who sends you an email, Google keeps a record of them in your Google Contacts. The unfortunate side effect is, if you ever sync your Google Contacts to another contact list (such as your iPhone address book), it winds up passing all of those one-time contacts into your address book by default.

Equally as unfortunate is that there's no good way to prevent it, aside from not syncing your Google Contacts with your iPhone.