I was recently very excited to discover what appeared to me to be a reliable rare STR signature match between two distantly related men in one of the lines of the prolific J2b-Z1043 lineage.
One of the men descends from a von Holtz who had migrated from somewhere in Germany to Sweden in the 1600s. He did a Big Y and is confirmed J2b-CTS11760 on the YFull tree as YF015631.
https://www.yfull.com/tree/J-CTS11760/
The other man has several GD 8-9 / 111 STR matches with the same surname variant Claunch / Clontz but the reliable paper trail ends in the US.
In the linked article below I explain my reasoning behind predicting that they may be each other's closest male line relatives based on the rare STR signature.
Relationship Predicted on Basis of Rare Shared STR Alleles is Wrong
A closer inspection of his Claunch / Clontz STR matches yielded the discovery that one of these men did in fact already do a Big Y and is not positive for the J2b-CTS11760 branch.
According to FTDNA he is positive for the J2b-Y12007 branch which makes him a relatively close cousin to J2b-CTS11760.
This would mean that their most recent common ancestor lived about 2600 years ago.
Unfortunately the email of this man who did the Big Y test bounced so we cannot analyze his result further or see if he matches anyone on the YFull tree.
So if the Big Y result is correct, and I have no reason to assume it isn't, then Holtz and Claunch/Clontz ended up acquiring the same set of rare alleles by chance rather than by more recent common descent.
The most shocking part of the coincidence is that the most stable of the matching rare alleles, DYS389ii-i, is off by two (18) from the modal (16).
For those curious why DYS389ii-i is considered as a single allele, it is because DYS389ii is actually the sum of two different alleles added together so makes no sense to compare.
Luckily I didn't use the term "smoking gun" when I wrote the article about my initial prediction. However it did seem like a very reliable prediction as I wrote.
Let this serve as a more extreme example of how matching rare STR allele signatures can be acquired by chance.