mongrel
I was watching The Chase the other day (yes, I know: virtually any other activity would have been a better use of my time) and I noticed that the presenter, Bradley Walsh, pronounced the word mongrel as it is spelt (a spelling pronunciation, as it is known) – that is to say with the first syllable rhyming with con. This pronunciation is quite common, although it is not the traditional one. The traditional pronunciation is of course mungrel.
Why is this? Well, I do happen to know the answer, because I learned it at university when I was doing my English degree. Long ago, before the printing press was invented, manuscripts were copied by hand, usually by monks. Their calligraphy involved the use of minims – that is, downward strokes of the quill for straight lines. But when you got a lot of minims together – eg when you got a u next to an n or m – then that was quite difficult to read. It looked like a bunch of four or five (or seven, if the u was sandwiched between an n and an m) downward strokes all crammed up together. So to get round this, when they came to that letter combination, they used an o instead of a u. So words like mungrel, as it would then have been both spelt and pronounced, came to be written as mongrel. The same is true for monk, incidentally.
Brandon Robshaw and the English Language 
Interesting explanation. There are loads U for O words which no one would mispronounce; cover, glove, month, honey, shovel, dozen, brother, etc.
That’s true. Good point.
As Simon says, an interesting explanation but it made me think why number is not spelt nomber?
Yes, good point: especially as ‘number’ must come to us via the French word ’nombre’. I have no answer, I’m afraid!
In Britain we do use No. as an abbreviation for number whereas in America they tend to use the # symbol.
Ah yes, Simon, but No. for number is a contraction of numero, I’ve heard.
Blimey, you did learn something!