ive noticed this to, but its only on the tick after you reach the value you need to cast the spell...

Say you need 40 mana
You sit and tick from 34 mana to 43 mana.
You attempt to cast the spell but it gets interupted saying that you dont have enough mana
You check your mana and you are sitting at 34 mana again.

Im sure it has something to do with client and server side mana values not sincing up right. To remedy this I med on tick past what i need. This can be a problem however if oyu need just one more tick to finish a mob and lose it.

This is because are trying to cast too fast.

Wait, you will tic from 34-43-42 mana. Once you lose that 1 mana and go to 42, that mana is solidified and you can cast spells using it. I think this was done to prevent kiters from running 5 seconds sitting for 1 second and getting a full tic of mana, you can still do it :P, but you have to time it more properly and it involves more sitting time.