Factorio Forums

If you shift+click a stack of ingredients from your inventory into an assembler, you are limited to 136 items (or whatever depending on assembler, etc). Manually clicking on the slot with the stack of items gives the normal error "Cannot insert more items into this slot."
If the ingredient in the assembler is one less than the maximum (in this case 135), you are able to insert the entire stack from your hand by clicking the slot, giving a maximum of 235 items. I have checked and this happens with assemblers and foundries.

Not a bug. The assembler "calls" a certain amount of items necessary for the recipe (twice minimum, but also influenced by its total speed IIRC), but one can add manually until full stacks of items are inserted.