This has now happened twice: when my pool was on during the day, I switched it over to spa and used the spa. I then turned off the spa and the pool automatically turned back on (so far, so good). But, tnstead of the spa filling and overflowing to the pol over the waterfall, the spa drained into the pool faster than the water got pumped in, until the spa emptiey or at the pool level. I then shut everything off. The first time this happened, the builder told me that there was a defective part. That was about a year ago.