If the citrus fruit has a swollen stem end, it's called "sheepnose" and it's a swollen nipple or blossom end, it's also called "sheepnose".  A swollen nipple is much more common. The fruit also tends to be coarse and the skin thick. Both terminal end conditions are thought to occur from too much vigor, low fruit numbers on a tree, high nitrogen, high temperatures at fruit set and early fruit formation and tends to occur more commonly with certain grapefruit and lemon varieties and rootstocks.   Overlay that background with general tree care associated with irrigation and pruning and it's not always clear what the overriding factor is that creates these fruit.  So then what brings all these factors together and how to prevent the condition is not at all clear.  It doesn't appear every year in the orchards where it is occasionally found. 

Tangelos and dekopons naturally have that necky stem end, and that's how you know you got one.

'Minneoloa' tangelo