Memorize Transactions feature is not used.
Most businesses have recurring expense every month, such us rent, utilities, marketing, etc. Memorize these recurring transactions to reduce data entry.  You can also memorize invoices, journal entries, and reports. This is a detail explanation on how to memorize transactions.

Not using Online Banking/Bank Feeds feature to download transactions.
Online Banking or Bank Feeds feature allows you to download transactions from your bank account, credit card, PayPal, etc. automatically into your QuickBooks. If you have entered transactions before downloading it, QuickBooks can match equal amount automatically. If unmatched, you have to add it into QuickBooks. Here is a detail explanation on downloading online banking transactions.

Not setting date/password annually to close books.
At the end of your fiscal year, once your books is reviewed by your accountant for tax reporting, close your books by setting date (end of your fiscal date) and password to prevent users changing last fiscal year data. QuickBooks will warn any users if this happen and only the password holder (administrator) can change the data. This is very important because unintended change can affect your financial.  When you get year-end adjustments from your accountant, the administrator can then enter the adjustments.

Not using “Ask My Accountant” account.

When facing unusual transactions and not sure which account to use, use “Ask My Accountant” account. This way, this account is a reminder that you have to recheck all transactions and/or ask your accountant, preventing you to make possible initial mistake and then forget to rectify it.

Use older-than-three-year version of QuickBooks.
Intuit does not support any versions older than three years, meaning that if there is a data corruption and you can’t fix it, then neither can Intuit. Because QuickBooks is inexpensive, the least is updating it every three years.