One of the bigger mistakes we made was not getting my Dad committed when he started losing his mind. We took his drivers license from him and his Doctors told him that he couldn't drive anymore, and I sabotaged his car, so that worked at keeping him home. Before that, he drove to town and didn't come back for three hours. It should of been 45 minutes. He didn't know where he went, he denied he was gone that long, and we realized that we had a potential problem.
Once he was stuck at home, he started calling people. He would get a wild idea, get on the phone to make it happen, and not tell anybody what he was doing. People showed up to take him to other states. He decided that Stanford had a cure for Dementia, but it was kept secret and the only way to get them to treat him was to go there in person. He called friends to pick him up so he could rent a car in another state. He figured that he would outsmart us that way. He booked airline tickets that we had to cancel. We took his wallet, but that freaked him out really bad, so we gave it back to him, but canceled all his cards.
Anytime he got a crazy idea, he acted on it. Most of them we where able to stop. But just yesterday we found out that he made changes to his medical insurance that now affects my mom. We don't know how bad it is yet, but it's not good.
It was hard to realize that his brain was gone, and he really had no idea what he was doing. Before he got really bad, he would have periods of sanity, and his biggest concern was to not be put in a home. We agreed to that, and it never happened, but it was VERY TEMPTING.
Another thing that he did that we didn't know about was messing up the Will. Fortunately, nothing permanent was done since my Mom is still alive and the lawyer was able to fix it, but it still cost her a couple of grand to undo his mistakes. It wasn't like he was trying to give anything away, he was more focused on keeping everything after he passed away so nobody would take it from him. Paranoid about money was a huge issue.
Once it happens, it only gets worse. Having a solid plan, and everything done and in writing for what happens afterwards is very important. Dealing with his stuff is easy, getting everything into Mom's name has been the challenge because of his never ending changes.