This is a fairly significant one.
Started a new 3.0 game, just to test it out. Set up my race as usual, fast-oob'd a small fleet, and got started surveying the system. I realized I didn't have any freighters, so I designed a standard freighter design, and retooled a SY to build the new freighter design. I then selected a construction task for that SY, and hit the "Add Task" button. The SY had two slipways, but by accident I hit the button three times. I noticed that the available slipways said "-1" at that point, and when I checked the shipyard task screen, it showed three freighters under construction. When I advanced the time, all three showed 2.4% complete, so work was completed on all three, in spite of the fact that there were only two slipways.
I tested this by selecting a construction task for the same shipyard, and hit the add task button a bunch of times, so that the available slipways ended up as -15. This was too many ships for the task screen, but they all seem to be worked on when the time advances.
Kurt