Here's a story from 2004 about a school IT tech who installed SETI@home on the school machines. The increased electricity bill caused some consternation and he lost his job.
I did that on some university servers around 1999. We had a few Alphas dedicated to batch processing, and I set up S@H to run on them.
One catch was that the duration of batch jobs was limited to 12 hours, so I couldn't just fire and forget. I ended up scripting it so that when the job ended, it automatically resubmitted itself to the batch queue.
It took about a day before I got a surprisingly friendly e-mail from the sysadmin asking me to please stop.
http://www.techdirt.com/articles/20041010/2225204.shtml
I imagine that mining crypto currency without permission is due for a high profile court case.