I had a problem with Leopard recently where I tried to update from 10.5.6 to 10.5.8 using the combined update and it failed repeatedly. I had to search for the 10.5.7 and 10.5.8 standalone updates and do it that way. Imagine my horror when I had to do it again on another machine and saw it had the original 10.5 on it. Fortunately the 10.5.6 combo update worked and I was then able to go to 10.5.7 and 10.5.8 as before. The only other times I did OS X updates was bringing my snow G3 iMac up to 10.2.8 from an older version and updating my Digital Audio from the 10.4.5 Tiger it came with to 10.4.11 as each update was released.