The obscure Russian nicknames! How is anybody supposed to know without being told that Sasha and Alexander are the same guy? (I do realize that while some English nicknames like Johnny for John are pretty self-explanatory, other like Jack for John or Dick for Richard are as opaque to foreigners as Alexander/Sasha)
Sasha and Alexander isn't that obscure thought. Very common example The real obscure diminutive for Alexander is Shura :D
The first time I worked with Polish people I had this problem a lot until I noticed the pattern. Someone told me to go and talk to "Maciek", it was only on asking Maciej where to find him that I found out they were the same person
The dialectal form Aleksasha (following the common pattern Mariya > Masha, Pavel > Pasha, etc.) might reduce the confusion somewhat.
I was in my 20's when I realized Bill and William are the same name