Why would one of my invoices say it was deposited? I never clicked the button. How do I tell QB to UNdeposit an invoice???

Let me guide you on how to 

It's possible the Automatically apply credits option is turn on in QuickBooks Online (QBO). This feature automatically applies credits to the next invoice you create for the same customer. 

You can turn it off if you don't want to automatically link the payments to your invoices.

Here's how:

1. Click the Gear icon on the top menu.
2. Choose Account and settings.
3. Select Advanced.
4. Go to Automation and tick the Pencil icon. 
5. Turn off the  Automatically apply credits option.
6. Tap Save.
7. Press Done.

If that option doesn't work on your business, we can manually unlink the payment posted on the invoice. This way, the Deposited status will be removed from the transaction.

Here's how:

1. Go to Sales on the left side menu.
2. Choose Invoices.
3. Open the invoice with the Deposited status.
4. Click the 1 payment made ( amount) link under the Payment Status section. 
5. Tap the Date link.
6. Uncheck the invoice in the Outstanding Transactions section.
7. Press Save and close. 

Just in case you'll receive bank deposits from your customer, check out this article to link them to an invoice: How to link a deposit to an invoice. 

I've got a link here that provides you with articles about managing customer invoices, sales receipts, and other sales forms: https://quickbooks.intuit.com/learn-support/en-us/sales-and-customers/sales-transactions/08?product=QuickBooks%20Online.

That should point you out in the right direction. Assistance is just one click away if you need a hand with creating recurring invoices or any QBO related. Have a great day ahead.