Shoppers are frustrated by seeing those empty shelves, and wondering, simply, why?
"I have no idea, especially on paper towels. It just boggles my mind," said Patrick Prouty. "And bottled water. I don't think we're having a water shortage right now, but people are buying that by the case."
WATCH: Shoppers clear store shelves of essential items amid fear of another COVID-19 lockdown
Walmart released a statement saying: "We are seeing pockets of lower than normal availability in some communities on bath tissue and cleaning supplies, depending on what's happening in the local area. We will continue to keep a close eye on product availability and work with our supply chain to help meet customer demand."
And from Albertsons, Vons and Pavilions: "We have asked customers to respect the quantity limits of select, high demand items like cleaning supplies and paper products to ensure more customers purchase the products they need."
The California Grocers Association is urging shoppers to take advantage of online delivery options.
The association, which represents more than 6,000 food stores and grocery supplier companies, said shortages of items like toilet paper shouldn't be a problem, but if it does occur, people shouldn't panic.
"People are starting to worry a little bit, and overbuying really is a result of people worrying -- that's simply all it is," said Ron Fong with the California Grocers Association.
Other tips for safe shopping include keep a distance of at least two shopping carts, consider shopping during off-peak hours, inspect items with your eyes instead of your hands and treat store employees with kindness.
MORE: Some SoCal stores seeing shortages of toilet paper again