I recently stumbled onto a Magento bug involving the currency switcher while working on a site for a client. The currency switcher was enabled and working perfectly everywhere on the site except the home page. Any products shown on the homepage (such as in a New Products block or widget) remained in the original currency.

After a bit of digging, I realized it was a cache issue, since disabling Magento's "Blocks HTML output" cache brought the currency switcher's full functionality back to life. It seems the products on the homepage were being cached as a part of the home HTML block, and therefor did not change with the currency switcher.

Of course, it's best to leave Magento's "Blocks HTML output" cache enabled to improve site performance. So what's the fix? Add a "cache_lifetime" node on your new products block and set it to the minimum allowed (1 second). In the end, your block code should look something like this:

{{block type="catalog/product_new" name="home.catalog.product.new" alias="product_homepage" template="catalog/product/new.phtml" _productsCount="8" cache_lifetime="1"}}
This will effectively keep the New Products block un-cached while safely keeping all other HTML block elements cached as usual. And if this issue is fixed in a future release of Magento, this fix can easily be removed by simply deleting the cache_lifetime="1" from your code!