Mine overflowed many times. It was an in-sump configuration, so it wasn't the end of the world... Sometimes I could understand why it did it, other times it just did it for no reason. It was also prone to doing the converse---stop making any bubbles whatsoever. In both cases, if I just left it alone, it would start doing its thing again within a few hours. The other 99.5% of the time it was a great skimmer, especially for what it cost new.