This happened to me, and I still shudder when I think of what the result could have been.
We had taken over a ship which the company had just bareboat chartered. The Chief Engineer had been allowed on board the day before and had gone over the vessel with the off going chief, checking the soundings and signing for the fuel and L.O. remaining on board. The rest of us joined on the day of sailing. As you can imagine it was a work up as we joined in the morning and sailed in the afternoon. No hand over.
Anyway we got away OK. Left Florida bound for Southampton. The day after leaving I decided to top up both main engine LO sumps, which were below the minimum levels, and alarming intermittently as the ship rolled. The LO storage tanks were double bottom tanks and were both recording full when sounded.
I lined up the transfer pump from the tank to the port ME sump. On starting the pump, something made me open a sampling cock on the discharge of the pump. A jet of water shot out. I hit the stop button on the pump with my heart pounding. Luckily only a few litres had been transferred.
Further investigation found that the tank was full of sea water with a layer of L.O. lying on the top. When the tank was sounded, the tape went through the L.O. and thus when wound back up to check the sounding had a nice coating of oil on it, giving no indication of anything amiss. The tank was tight with no leakage to adjoining tanks or cofferdams or to the sea. There were no connections to any sea water (or fresh water) lines. The only way that water could have got into the tank was a deliberate introduction down the sounding pipe.
Whether the off going crew in a fit of anger at losing their jobs had filled the tank with water or whether they were trying to hide from the chief engineer the fact that they had lost a lot of oil in an unknown incident I'll never know. But for opening that test cock, one engine would have certainly have been ruined, maybe both if I had transferred to the starboard sump before the port engine failed.
Again it is easy to be wise after the event. But I'll tell you this; Every time I transferred L.O. to the main engine I would sample what I was pumping, and offer a silent "thankyou" to who ever was watching over me that day!