I don't have much credit finding this weird thing but I wanted to know if it's a bug or if it's really intended, which I cannot imagine. 

copy paste from this blog: http://blog.tquila.com/2012/06/25/grokking-force-dot-com-apex/

"Unit Tests increment your Objects unique Id

This is not covered in the documentation, or atleast I haven’t found it in the Apex docs. When you run unit tests that insert/create records for a particular object(standard or otherwise), it increments the value of the Name field(of type Auto Number) outside of the test context.

Let’s say you have a custom object “Invoice Statement” (Invoice_Statement__c) that has a Name field called “Invoice Number”, and is of type “Auto Number”. The format of the field is: INV-{0000}, and let’s assume that the latest record number is INV-2058.

If you insert 10 records in the test class as follows:

Now, when you insert a new Invoice Statement record outside of the test method via a trigger, or batch, or the User Interface, the next inserted record will have the id INV-2069. So don’t rely on the unique name field if you have strict rules around the auto-increment feature in your application. Instead add a custom field which is auto-incremented in a workflow or trigger to have more granular control over how the value is incremented."

I verified that by myself, and this indeed happen. I remember having used these numbers for customer who want to have exactly one unique number per record (especially for bills), so this could be a huge issue.

Anyone aware of that? I didn't find it in the Salesforce known bug ....