Hi guys
I was looking at building my arena today, entered some lower tier, then decided against it. Change the number to zero, then pressed "Get a quote". 
(that is to say, i have Zero, in all the fields). 
I get returned with the following:
Expand arena  
Requested Expansion:  0 Bleachers 
 0 Lower Tier 
 0 Courtside Seats 
 0 Luxury Boxes 
Total Upgrade: 0 seats 
Total Cost: $0  
This expansion will take roughly 6 days. 
 Now, i dunno about you, but zero seats, for zero dollars, should probably not take 6 days. 
The 'Yes, Build it!" Button was also active. I didnt press it, but i can only assume that i would not be able to build for 6 days after this. 
More a bug in case a user accidentily doesn't type the number in or something, and i can only assume that they will be locked out of building for 6 days -so its probably worthwhile just having a little 'check for all seats equal zero' test, and if true, don't make 'Build it' button active - or something like that.