This is mostly a post for UK readers, but it's probably worth monitoring for similar situations elsewhere in the world.
In a move which has received very little publicity, Royal Mail are introducing barcoded postage stamps. This ties in with the labels you can buy online or at the post office, which have had barcodes for quite a while.
Barcoded stamps can sometimes help with tracking (although normal letters only get a delivery scan, not full tracking), but Royal Mail's main justification is that it will reduce fraud. Apparently there is a problem with people removing used stamps from envelopes and re-selling them. Royal Mail appear to have stopped the old-fashioned but effective method of rubber-stamping stamps on envelopes at the sorting office to indicate they'd been used.
The problem is that for decades, Royal Mail have been selling stamps, loose or in books. It's a hugely convenient way of sending a letter; just put it in the envelope, take a stamp from your pile of them in the drawer, stick it on and pop the letter in a nearby postbox.
That means there are millions (probably tens of millions) of stamps sitting around that people have bought and paid for. Actually, until now stamps just showing the class of postage (1st Class, 2nd Class etc, rather than a cash value) would be one of the best investments you could make; Royal Mail price increases have been significantly above the rate of inflation.
Here is the key thing - AT THE END OF JANUARY ALL THOSE STAMPS BECOME UNUSABLE AND WORTHLESS.
Royal Mail have not publicised it at all well, but there is a page on their website about it - https://www.royalmail.com/sending/barcoded-stamps - on there, you can download the forms you will need to use to return old-style stamps to them.
You need to act now, to get the stamps back to them before the end of the month.
Personally, I think this move is shenanigans. They are only swapping the stamps over, not offering to refund the cost. More importantly, with a lot of people probably being unaware of this, Royal Mail will be able to write off millions of pounds worth of suddenly useless stamps, which will make the accruals on their annual accounts look massively better than last year !
Edit to add: photo is by me :)