If you have assets that you own jointly, whether it is with a spouse, a child, grandchild or someone else, that asset will typically pass automatically to the surviving joint owner. It doesn't matter if you have a super-duper-deluxe will/trust, the joint ownership designation bypasses your estate plan and potentially short circuits any plan you may have.
For some situations, holding assets jointly might be sufficient for transferring your assets upon your death. For example, a husband and wife with no kids from a prior relationship and with modest assets. However, consider the following scenarios and the unexpected result:
Scenario 1 - John Washington has two kids. He loves them both and wants to make sure they are treated equally with their inheritance. One of his kids, Chris, lives nearby and helps John out with payment of his bills and expenses. To give Chris some flexibility, John adds Chris as a joint owner on his bank accounts so that Chris can help out without John's signature. When John passes away, those bank accounts will go automatically to Chris and his sibling will be cut out. While Chris may choose to share the joint accounts with his sibling, there is no guarantee that he will share and it may end up that Chris receives more than his "fair share". Or, what if Chris has a judgment or tax lien against him? Chris' judgment creditor could garnish the bank accounts and take John's money away for payment on Chris' debt.
Scenario 2 - Wife and Husband are both in a second marriage with kids from a prior marriage. H and W both want to preserve some assets for the children of their first marriage. But if H and W maintain joint bank accounts, those assets will pass to the survivor, and then the survivor will be able to select who ultimately receives those assets, which may be their own kids to the exclusion of their deceased spouse's kids.
Reviewing ownership information is one of the key components to your estate plan. Be sure your estate planner has all the information on how your assets are owned.