"We were so scared, that's why we left early. I won't go up there again", said Lauren, 12.
Lee, 12, said: "Everyone got up to dance to Will and everything started rocking - it was scary. I just thought it was going to collapse".
But the Arena's General Manager told us the seats are designed that way and people shouldn't worry about the rocking.
End of the road
By the end of the UK tour, Will and Gareth will have performed to over 500,000 fans!
And as Will exclusively told Newsround on Thursday, the pair finally go their separate ways in the pop world after the tour. Boo hoo!