Aha! If it happens in the randomized buildings; that's an easy fix - hopefully. I'm probably just not bounding the random new objects correctly or some other logic flaw when creating random new enemies.
Well, I found the display bug, that's an easy fix. The multiple-bonus-items-in-a-row bug is more perplexing. Can you give some details? How often did it happen? Was it immediately after a death? When did it happen? After you passed the 15th level and start the random-enemy level(s)?
I could just replicate it for a dozen times in my game. It was not really related to death, I more got the impression that it happened when it wanted to do the bonus anyway. It started from the second randomized building onwards.