Yes, I get this too and have been trying to track down the cause. Frequently I get it and Activity Monitor is not even showing much of a load (though sometimes DCUM pages do show a lot of resource usage). I haven't able to find the cause but at least it doesn't seem to cause any actual problems.

Hi Jeff,

This is probably a Mac-only thing, but on Safari I frequently get "this website is using significant memory" or "this website is using significant energy". It is ad related, as you can probably guess, but it's such that it happens probably 30% of the time when browsing the messageboards and probably 80% of the time if I walk away from the computer for a few minutes and leave a page open.

Wanted to mention it to you in case you have seen similar. FWIW I do not run Flash on this machine so maybe the adverts are trying to call Flash and crashing, but I suspect it is some open Javascript or something.

Just FYI.