It usually is not a mistake, though a kit arriving without the batteries it needs certainly looks like one. Some products include the AA or AAA cells they need and some do not — the product page says which. Where a kit normally includes them, shipping rules can still change that. Some countries do not let us send alkaline batteries by air, so for those destinations the same kit ships without them and you buy them locally. That is why two people can order the same product and get different contents.
Check the box or the manual first. If it says "Batteries not included", standard AA or AAA cells from any shop will work.
Use fresh alkaline cells rather than rechargeables. Rechargeable AA and AAA cells put out 1.2 V instead of 1.5 V, which is not always enough for our kits to run properly. Never mix old and new cells, or different brands and chemistries, in the same device.
Built-in Li-Po batteries are different: those are always included, wherever you are. If a kit that should have a Li-Po battery arrived without one, message us here, or email [email protected], with your order number and we will sort it out.