Factorio Forums

When shift + left clicking to add ingredients into an assembler, the stack size inside the assembler can end up larger than normally possible. For example, if there are already 17 copper plates in a machine and you shift click to add a stack of 100, it overflows to 117 instead of maxing out at 100.

That's by design. Assembling machines can hold more than the normal stack sizes and even more so when using speed modules/beacons.