Last week, when I was writing a cheque I accidentally wrote the year as being 2006.
And now I just realised that I've spent an entire week telling people that I'm a fifth-year PhD student. No, this is my fourth year. I was always correctly saying that I've lived here for three and a half years, but then I'd say that it's my fifth year here.
I don't know what that says about the current state of my brain, but I'm pretty sure it's not a good thing.