I was dominating a huge map with many colonies. I speed built between 30-40 ships of my strongest type (huge ships, 75 HP) using all colonies. I had all the special buildings to increase HP of all ships. I was using "fast combat" (animation, but no battle screen) on the main map to speed up my expansion. There were more "ship built" blue icons on the right than would fit (might have been a factor). When one of my ships (from this batch) would repeatedly attack a colony and should have exploded, it was restored to full life, making it invincible! What's more is that at the same time, a blue icon would float across the right side of the screen as if I had built another ship! At least 2-3 of my ships were affected in this way, but after I cleared the icons a few turns later, newly built ships were not invincible (but the old ones still were). Wierd.