The industry bar will say "MAX" when that turn will build as many or more factories as your CURRENT population can handle. Let's say at the start when each population can handle 2 factories, you have 80/100 on your homeworld and 165 factories. The industry bar will say "MAX" because your population of 80 can only handle 160 factories.

However your BC do not disappear, it just means you're not getting the extra production out of the factories you don't have population to work. The game will never let you build more factories than your MAX population can currently handle; it will automatically stop and readjust the bar to research (the above example will stop at 200 factories). Any spillover will go to your reserve.

The only "waste" comes out of not getting the production out of those extra factories until your population fills out. The math on that gets trickier since I'm not sure if putting production into growing population is worth it (production wise).