Using Magento 18.104.22.168, I noticed an interesting bug this afternoon. If you have setup any Date & Time or Time custom attributes, and have made them NOT required, here’s something you may notice:
When initially adding the product to the cart, and you leave the Date & Time or Time custom options NOT filled out, your product will be added to the cart just as you would expect. The untouched custom options are not applied to the product in your cart. However, in the rare case where a user is clicking the “Edit” link in the cart to edit it, and you still leave those custom options untouched, the validation will bark at you saying “Field is not complete” on those custom options. The only way to edit that product then is to put values in for those fields. Magento updates the product as it should, but you now have those other custom options in play, which could screw things up.
I have submitted the bug to Magento: http://www.magentocommerce.com/bug-tracking/issue?issue=12183