I've purchased a downloadable version of Rosetta Stone (Spanish) v5.0 recently. It can be installed on two computers simultaneously.
My Mrs. and I got the same copy of the installer, installed it and tried to activate the software. I managed to activate it within a second, but I kept getting a weird error code 8111 on my wife's laptop running the same OS X 10.10.3 as mine.
After spending a couple of hours with Rosetta's support telling me to follow the instruction I had already tried an hour ago, I was close to give up and ask for a refund.