I have run into it three times. With my first iPhone, it was several versions old and after one particular update all my extra storage was disappearing. ISame thing with my second, but it actually bricked on an update due to the FireWire slot being loose and the cable not having a good connection. I have not taken any pictures with my most current phone in the last several months, nor have I added music. But I'll be dipped if my current phone all of the sudden is out of space again. Last month, it wasn't even 65% full.
I have had to purchase storage space 2x and I barely use my phone for ANYTHING!!!