Once - with my first gerbil (I knew nothing at the time about them) the man on the stall said it was a male. It was the only one left and we needed to get it a companion. So the gerbil and I went to the local rescue centre - no gerbils but they did double check what sex it was - they verified (2 people) it was a male.

So the next day we got him a male companion from another store and introduced them just fine. However the next day gerbil number 2, Nibbler, was very sick - my vet said he had Tyzzers which was highly contagious so he was separated from gerbil number 1. Nibb was only given a 5% chance of survival, my whole family helped me hand feed him every 2 hours and our lounge was like a sauna to keep him warm. By the time he was fully recovered and ready to go back with number 1 a month had passed. Re-introductions via a split cage went well and they were back together.

By now they were tamer and I could hold them, looking at their bits I could tell a distinct difference in the maturing gerbils - number 1 who was male according to 3 people was actually a female - my vet confirmed it. So they were again separated and Nibb the true male neutered. He and the "old cow" (Miss Piggy!!) have lived together happily for a year now.

It did teach me a lot, having one of each meant that I can usually sex a gerbil without even having to pick it up, the difference is rather obvious!!!! I was annoyed that the rescue centre got it wrong, but then I think it was fate that meant we got Nibb and he survived (my vet said the chances of any other gerbils in the shop surviving was nil).