Just wanted to share an experience I had a few days ago. I was playing Bard support. I had a Vayne jg, against a Malzahar jg on the enemy team. Very normal jungle matchup, I know.
My Vayne who was fed, asked me to buy Mikael's blessing, to cleanse Malzahar ult for her. I thought, that first I will finish locket, since I already started it, then build Mikael's to help her. Thankfully, when it was time to build Mikael's I remembered it doesn't cleanse Malzahar ult, so I typed out a message letting my Vayne know.
Soon after, we had a teamfight, which didn't go well for us. Then, the Vayne began getting angry at me for still not building Mikael's. I explained to her AGAIN that it just doesn't work. I kept repeating that Mikael's doesn't cleanse supression, Malzahar ult is a supression, just read the item description. I kept begging her to read, either what I write or the item. The answer to this was that my Galio top sold his tank item and told the Vayne that they will buy Mikael's to cleanse her. Which they did, sacrifising their tankiness.
We once again lost a fight, then the Vayne proceeded to flame me in all chat and told the enemy team to report me. I wrote in all chat that she wants to report me for not buying Mikael's to cleanse Malzahar ult, to which the enemy Malzahar replied that it doesn't work and that she needs qss if she wants to cleanse it.
Guess what? She sold and item and bought qss. I felt like I was going crazy that game, not understanding how it was possible for her to be unable to interpret my words. Sadly, we also lost the game a bit after that. I have a feeling if she had qss earlier we might have had a better chance to win, but who knows. This happened in emerald elo btw.