What a relief - I'm not the only OCD sufferer here! I do a numberplate thing where I have to firstly, try and anagram it. If it doesn't work, I have to make a word like Babyjane does.
I also have to anagram the letters on manhole covers, and work out patterns with telephone numbers on the keypad. So I like numbers that only use the 4 corners, or down one side, like 1471. Drives me mad.
But the worst is one that I've had all my life and it's a bit of a party trick too. Someone can give me a word, and I'll repeat it instantly, backwards. This one takes over if I let it...Someone could be talking to me and in my head I'm reversing everything. I read signs, posters on buses, anything, forwards then backwards. My brother does a similar thing where you give him a word and he tells you how many letters in it - without really thinking.
Cockney_si - you've opened a can of worms here - can't believe so many of us do these things, but it must be a sign of intelligence, surely??!!!
:0)