On-premise catering has a kitchen to fall back on. Off-site, if it's not on the truck, it's not at the event - and the items that get forgotten are rarely the big ones like plates and napkins. They're the small, easy-to-overlook items that only matter once you're already set up and realize you're short.
Serving utensils in the right quantity for the number of chafing dishes and platters you're running - it's common to pack enough chafers but not enough matching tongs and serving spoons for each one to be staffed independently. Extra trash bags beyond what looks like "enough," since off-site venues rarely have their own on hand. Sternos or fuel for chafing dishes, packed with a spare beyond the calculated burn time in case service runs long. Table linens sized for the actual tables at the venue, not your standard size, if the venue's tables differ from your usual setup.
Cocktail napkins separate from dinner napkins, cups sized correctly for both non-alcoholic and alcoholic pours, and enough ice-transport containers to move ice from your truck to the bar setup without it half-melting in transit.
The items that get forgotten are almost always the same handful, which means the fix isn't "be more careful" - it's a checklist that's specific to your setup and gets checked against every time, not recreated from memory on each job. Catering Calc Pro's disposables calculator builds this list from your guest count and service style automatically, and it saves as part of your event template so the same job type doesn't require rebuilding the list from scratch.
Plug in your guest count and get exact numbers in seconds.