Originally Posted by
Bill Kapaun
IF it's jamming between the 11 & 12, you might be missing a shim that would go between those 2 cogs.
Sometimes they use a "generic" 11T and toss in a shim to get the proper spacing.
That way, for example, they can use a 9 speed END cog for an 7/8 speed cassette.
Yes, this can be an issue, but it's not consistent with the facts as reported. The OP days that when he put the original chain back on, all was well. Assuming he didn't work on the cassette at the same time, and make a mistake, then magically correct it without knowing, that makes the chain itself the only variable.
Not having any more info, except that the chain in the box was a different brand than the one on the label, I'll venture that the OP might have been the indirect victim of a retail return scam. Some one buys the chain, then returns it after switching the chain to a used or cheaper model. The vendor accepts the box, credits the buyer and puts it back in stock, without checking the contents.
Of course, that could be fanciful nonsense, and the OP might have had this chain sitting around for eons and forgotten what he put in the box.
BTW- the OP might also have a spacing issue, not bad enough for the right chain, but enough to make a critical difference with the wrong one.