Hello WickedNag (what a great sign-in name)!
Unfortunately, yes, we have experienced a break in on our portable electric fence.
While camping in the Uncompahgre Mountains of southern Colorado in September 2009, we had our boys corraled in a 200' by 200' portable electric fence setup. Late in the evening, just before we were scheduled to move them to a highline, a small herd of Elk came through our campsite. One or more of them apparently became startled (we do not know why - the horses movements, a bear, us, ???). One of them bolted through the electric fence, received a shock, and abruptly did a reverse turn out - the same way he/she came in - over the knocked down fence.
Since we were in the immediate area, the liability was nil, but the potential for damage or escape by our boys was real enough that we knew not to keep them in the electric fence except when we are present in the campsite area and certainly not during the night.