In my opinion, the super-giant suitcase is a rookie mistake, and one that I've paid dearly for a good few times!
I feel a tiny bit insulted by that as I'm no rookie, I've made this 5,000 mile flight six times!
I bought the offending suitcase back in the days when its measurements actually did comply with, I think it was, British Airways size requirements, if I recall correctly. I bought it in 1999 but never used it until my trip this year, by which time not only was I flying a different airline, but I believe many had pulled their limits downward an inch or two if I'm not mistaken.
Also, I did indeed "research my airline" --- over and over again, double checking on not just my own baggage requirements but that of my cat's kennel etc. I had the added aspect of bringing my cat under the PETS scheme to concern myself with, a higher priority that worrying too much about my baggage. I knew before packing that the case was possibly going to be oversized, but I wasn't sure which way they would measure it -- believe it or not, it came up different for me on several measuring sessions. I do not know quite how I managed that, using the same tape measure, but.....
So, I went to the airport already knowing I would be looking at an oversized penalty charge. The person who dealt with us was VERY nice even though he could not in good conscience waive our penalty. We too were very nice to him -- the whole encounter was as warm, and friendly as it could be, and the man was even extra helpful by getting for us one of the airlines duffels to solve at least one part of our problem. He went the extra mile just to spare us one of the penalties and we were very happy with that, as we knew going in that we'd be dealing with at least the other. Even though we still had one penalty to pay, the entire moment was all conducted with goodwill on both parts, no lack of politeness.

However, on the weight issue, we were using my bathroom scales and things were very frantic that morning. We did the best we could and figured we were actually just
at the weight limit. Admittedly it was not a priority at the time. There was A LOT going on, that final morning. I had a scared cat whom I actually had to cut out of the back of my sofa with a bread knife, in order to get him into his kennel. He tore up my arm and I was bleeding all the way to the airport.
I had been up literally all night packing and realizing I had kept back too many "essentials" from my freight shipment and it wasn't all going to be able to be packed
without violating one of the baggage rules. As it was, I still had to get a friend to sneak into my no-longer-mine apartment and pick up the things I had to leave behind.
Remember, this was not a "returning from vacation" or going-on-vacation flight that I was packing for -- this was a full one-time-only "moving countries" flight I was packing for -- there's a huge difference. I was going home for good on this flight, with no further trips back and forth possible (I was closing up a rented apartement and I don't intend to visit anyone there again) and was trying to bring everything that meant anything to me, family heirlooms included. This was my one flight, my one shot to take all my things. No repeat trips.
These things were taking place even within the best possible planning I could put into this operation. I was moving back home to the UK after nineteen years living in the US. I researched like crazy, and with no help, either. I had to orchestrate everything about this re-emigration by myself, and it nearly killed me as it is. I'm lucky MORE things didn't go wrong. The suitcase was the ONLY thing that did not go 100% smoothly. I'm not a slipshod, careless person. The stress almost made me psycho. I'm not used to this kind of disruption, and when I did it the first time I never wanted to do it again.
But please don't call me a rookie. I've made the UK-US trip six times in my life. For someone who considers herself a homebody, that's a fair amount to clock up. And I don't intend any more. I've also now emigrated and re-immigrated twice in my life. I don't intend to be flying around the world like this again and I don't intend to need to ever bring as much in my luggage as I was bringing this trip. This was me trying to move all my worldly goods, including things I did not want to trust to the ocean shipping, back to my homeland, not just a vacation.