by Tony Levi.
Well, we run a pile of big sites and just let their crons run concurrently - it doesn't really cause any problems for us, with a handful of needed patches.
It is necessary in some tasks to set the "last run" time at the start rather than end of the process, so the next run won't attempt to do the same thing right away.. forum digest and statistics come to mind. It would be nice to see better/more use of transactions/locks with concurrency in mind though.
Haven't seen any problems caused by out-of-order execution, so I don't think this is a big deal.