Things you don't see because you're not expecting them
One of my colleagues lives in High Wycombe which is in Buck and not far from where I grew up (and indeed not that far from here because I grew up not that far from here). We have chatted numerous times about local schools / what her son's doing at school etc.
Also Mr Cloudhigh (whom God preserve, of Utrecht) and I went for a tour of our old school - I think they were after money but fortunately they only had HIS address :-)
In the course of neither of these, until today, did I realise that the age at which one transfers from middle school to secondary school has changed since I went to school. In my day one's first year at secondary school was the year one had one's 13th birthday. It is now the year before that, what the hip young people call 'year 7'. In old money this would be the 1st year. In my day, the first year of the secondary school was the 'second year', because they hadn't renumbered them when they had previously hanged the year one started at the secondary school.
This emerged because she said that her son needed to choose a language for the next two years, prior to choosing his GCSEs. Hang on, I thought, surely that is one year not two.
A few years before I started at the secondary school they changed the year from the year of one's 12th birthday to the year of one's 13th - essentially because when the school leaving age went up to 16 from 15 they decided to absorb the extra year in the infant school.
I am sure that this should have been obvious from previous conversations and our tour of the school if I had thought it possible that it would change.
The best example of people not seeing things they are not expecting was when someone at work said 'I thought you said you were going to bring in the giant penguin' when the giant penguin was standing about 5 metres from her. People tend to stop noticing the office penguins after a while.