Perhaps it's dementia, but check his reading glasses as well. If your vision gets slowly worse, you don't necessarily realise that you need stronger specs.
My Mom was very fastidious with her finances and kept track of all her bills on an old fashioned ledger. When she started to make mistakes on it and her perfect penmanship got a little shaky, looking back now it was the first indication that she was having problems. Your husband could just be messing up like we all do occasionally, or it might be something else. It’s good you’re checking his work, but I wouldn’t panic yet unless you’re seeing other indicators as well.
