Tax.
I've been going back. My wife was financially qualified so she used to do it, but she got it wrong, I found as soon as I looked at it.
It's 6 months since she died, but I'm still not up to date with moving accounts about.
Once again, I found HMRC surprisingly helpful.
I thought I'd ring them to say look, I know this is wrong and I'll owe you interest and the fine, so can I pay a few quid now on account?
No need they said, because they had a discontinuous record. In this case that's a euphemism for telling me I was dead, when she died.
The surprising bit though was that when I said there was a lot to do, they gave me a list of all the accounts they knew about which had paid me interest. Account numbers and interest. There were 13 of the bloody things! I had no recollection there were that many. I phoned a couple of places to check the biggest, and they were right. Saved me hours and loads of angst, I can just put in the figures they gave me.
(Oddly, they are NOT routinely informed of capital gains and dividends - you have to tell them.)
So when I get round to doing 2025-6 tax, I'll phone and ask them what they have.