Yes, and/or that they were in a hurry.
Or just messing with people. Sometimes I write below the line, in the different space where address should be, put return address on back (Euro style), and also put novelty stamps (send without my consent to me from charities begging for money) in addition to real US post stamps not on area, etc. Unless it is an important letter, bill, RSVP, etc where getting delivered is totally desired- why not mess with them? And force person-handling versus machine handling? In a weird way I'm supporting the postmen and their jobs! I mail packages sometimes using MANY old stamps, technically legal.
ps: my personal protest against zip codes has been ended and things will be returned-to-sender or lost if one does not use. As all knows ZIP codes was a Soviet plot, hahaha... but the not using ZIP-Plus (which id's the individual address, basically,) still works. Also, technically and legally, you can still use "General Delivery" however the USPS has limited it and, if in a large town or city, usually only one place that can receive and will hold "at postmaster's discretion."