Don't be daft, occasionally thing will happen with these engine things. If you think about it, to really be sure about working on an engine, you would have to use clinically clean conditions, etc to make sure no possible contamination got in. But , hey this is life we dont have access to that environment, so we do the best we can. If it threw a rod, possible causes are bad lubrication of bearings, or the rod just gave up. Unless you power flushed all oilways, you ain't going to know if there is some crap lurking there about to break free and block something. it might have been on the way out before you got it. Re the plugs, sod torque wrenches, I always tighten up hand tight plus a further 1/4 and it works for me.
A thing to do is to examine it and work out what went wrong. That way you will learn from its demise, take corrective action in future, and it won't happen again. Its called learning from mistakes. I did just the other night. I bought some "heavy duty racking" from crapbay, "designed in Germany". Oh how I fell for it.
If you imagine the old bolt together rack from Dexion, it could be pushed, bent, overloaded, stood on etc, and would still work, no distortion, and even if you really overloaded and bent it, it would still work. This crap stuff clips together, and provide the load is directly on the top, ok, but any sideways movement and the clips bend and the whole thing requires major rework to sit straight. So I learned a lesson, no more of this clip together rubbish, Dexion rules.
I'm sure you are still good with engines, and would trust you more than the "Pink Pair". Would you let a hairdresser near your car ?

Ken