Korallia did a modification for the pumps that would start up backwards. It was a new front cover with a rubber piece in it. If the motor starts backwards, the impeller slams forward and the blades hit the rubber piece and stop. If it starts forwards, the impeller is pushed backwards, which is the intended thing for them to do. I have a wavemaker on some korallia's and they sometimes will knock the front cover off at startups.
Another problem I have had with them is that the impellers will go bad. The magnet splits, making it larger diameter, which makes it rub inside the casing, or simply lock up. I think this is caused by the material rusting on the inside of the magnet, causing it to expand. They have typically lasted a couple years for me before doing this, but I haven't actually put them on a calendar to make sure. Last time I had one do this, I went to Korallia and asked for a cheap source to buy them, and they mailed me a couple free of charge. Pretty nice!!