I found this in a Coinstar machine along with a bunch of unremarkable change that I will not mention here. It is a 1943-D steel penny, in decent shape.
I believe the coin star is rejecting things that are off metal, more so than the weight. Because it accepts both copper and zinc cents even though the weight is different, but it rejects the steelies which is in between both those weights. Likewise why I think it is rejecting silver dimes and quarters. Because of the metal and not the weight. As for the foreign stuff, mostly for size but also for metal. (Just a theory.) Also, coins that are the correct metal but damaged get rejected. And it seems when it spits that one out, it will shoot out some other normal coins behind the jam as well. (Or if they dump in too many at once and it clogs the mechanism.)
Well, for starters, no US coin machine is going to accept anything that's magnetic. Steel slugs -- electric-box punch-outs, washers, and the like -- are probably the most common thing you'll encounter that's roughly the size and shape of a coin, but isn't one. I think most mechanisms also detect the different performance of different metals as they go by those magnets and experience eddy currents. So, you're right, they probably do make an effort to reject off-metal items -- but they'll reject things by weight, too.
The Bahamas quarter used to be the same size and weight as a US quarter. Panama too and their money is 1-1 with US. Perhaps it was rejected for metal content cupro-nickel. The 1955 UK 2 shillings sounds nice. Photo?
Here's the trio. Saw that it was big and worn and was hoping for a Franklin half. The florin is clad but still an unusual find.
The 2 shilling is nice. They used to have silver (up to 1946?) but that coin is cupro nickel. NGC lists it as 50 cents in VF 20 and $2.00 in XF 40. Their prices are a little high, but that's a keeper.
Maybe it was only a cent. A nickel or quarter would be rejected. I got a really smooth (AG-03 at best) 1942-S nickel in there one time. That was awesome.
Now I'm tempted to start leaving scrapped 2x2s at the CoinStars around me, just to keep people guessing. I've got a few labeled for Morgan and Peace dollars...
And for the people who realize that CoinStars reject silver, perhaps a few nickel 2x2s labeled "1885 AU+++", "1912-S XF", "1937-D 3-leg"... and maybe a couple of small-cent ones labeled "1908-S" or "1914-D MS+++". If I went for "1909-S VDB", they'd know I was trolling.