I have two Oase filters, a 600 and a 250. I had a similar experience.Hi Anon, I was right on top of it with towels hopefully no insurance needed. It certainly could have been operator error, but I've done it so many times and if it's not lined up right, it won't lock.
These filters have a failsafe feature which will keep this from happening with one exception. In theory you cannot lock the input/output turret without the prefilter assembly being locked. The gotcha is that if you do not push the prefilter in securely you can still move the locking lever into the locked position. Then when you lock the input/output turret, which also opens the valves, water will siphon the tank out the now unsealed prefilter opening.
I have double-tap connectors on my input/output tubes but the real key is to always give the prefilter a tug after trying to lock it and before locking the input/outputs.
BTW. If it happens again just quickly unlock the input/output turret lever which will close those tubes and stop the siphon.