Can someone who unsubscribed still receive future emails?

If the email is operational, an unsubscribed lead can still get the email.  

What if it's no operational? I had a lead unsubscribe in February and they continued receiving non-operational emails...

That doesn't sound right. I would recommend confirming whether that person's Unsubscribed field is checked, or it's value is 'True'. It also depends on whether you're using Marketo's standard 'Unsubscribed' field to track opt-outs or whether you're using your own custom field.

The only thing I can think of is if someone set the lead's record in Marketo "unsubscribed" from true to false.  You can check the lead's activity record to see what happened.  

I believe if a lead unsubscribes, and then fills out a form later they are re-subscribed to your email content. Is it possible that they could have filled out a form to update their lead record?

Marketo has a built in global unsubscribe functionality that will automatically prevent anyone who is opted out from receiving non-operational emails. That said, are a couple reasons why someone may still be getting emails, even if they're "unsubscribed"

1. The lead is unsubscribed  from a custom field, like a newsletter subscription field, but not the primary opt out field. The system-controlled opt out field is called "Unsubscribed" in your Marketo instance, and it has two values: True or False.

2. The lead received an "operational" email, which has overriden the global unsubscribe settings. This feature can be activated for any email, but is really only meant to be used in instances where the email cannot be considered spam. You'd use this only in situations where the person is expecting an email, or needs critical info from the email, such as for an event registration or an autoresponder, or in certain instances, a critical customer communication.

Hope this helps.

Is there a duplicate record with the same email address in the database? That might be it.

Other than duplicate records, the most common reason for a lead that is unsubscribed to continue receiving emails is that they either have an alternate email address stored as a lead in your account, or an address that is an alias for that email address.  Sometimes a lead will receive an email via one email address, and mistakenly enter another email address in the unsubscribe form.  In the case of an email alias, the lead will receive the email to their primary inbox, but will not realize it went to the alias, and fail to unsubscribe that alias.

Ooh so true about the duplicate record.  I totally forgot about that and that has happened in my case.  

Thanks guys, but I've looked at every single one of those reasons. I'm stumped right now and creating doubts about Marketo's unsubscribe rules...