Has anyone ever mistaken a common stamp for a rare one?

Oh, I do it all the time.  See the design in the catalog and assume that is the stamp, not realizing the design was used again 10 years later.

I have done it a few times. I do my best to get my hopes up too high. I sometimes think I see what I want to see.
i always try to take it as a learning opportunity.
I collect fir the pleasure, so value is not as important. To find a genuine “treasure “ would always be nice.

David

Thanks for starting this thread, mberry.  It's an experience that most of us can relate to, I think.  It is especially disappointing, I have to imagine, to have a stamp that you previously took as a high-value item, only to re-examine it later and realize that it isn't what you thought it was.  Sorry about that, but I can empathize, because I have done the same thing.

Antonio's (smauggie) point about checking the catalogue for later re-issues of an earlier design is a good one.  I have been tripped up by that situation on more than one occasion.  I will at least give the much-maligned Scott Catalogue some credit in this regard, as they often put notes at the bottom of a listing that will state something like: "See also nos. XXX, YYY, etc."  That can be very helpful, and I have learned always to check those related listings, even if I think I have identified the stamp in question correctly.  It never hurts to see what they are referring to.

Aaron (kasvik), while I agree with you that Terri's (philatelia) find of a true modern rarity in a kiloware lot is a dream come true and does her great credit indeed, kudos absolutely well deserved, there is another thread that you might want to check in regards to the amazing finds of some eagle-eyed TSF treasure hunters:

thestampforum.boards.net/post/119413/thread

There have been several remarkable finds, but the most incredible of these is that of our member watermark, who bought a Canadian Large Queen for about $1.00 from a circuit book, got it expertized to validate that it was indeed printed on laid paper, and then sold it for $247k at auction, one of only three such stamps known to exist!

But if you read through that thread, you will also see that Alex (vikingeck), daniel, Mark (Mr. H), and Dave (DK) have also made fabulous finds in unexpected places.  Making these incredible finds is, of course, the exception to the rule, which is that most of us more frequently do exactly the opposite of this rather exclusive group, by thinking we have found a rarity, when we really haven't.

All of that said, I keep searching, and I hope to join the club of successful treasure hunters one of these days!