As Nick said the wastegate flapper arm will come out a bit and hit something causing high boost levels, or the opposite can happen and it won't align at all and cause low/no boost.
I have used a hammer(yes I know) and very carefully tapped the wastgate arm inward so that the spacer/sleeve (between the arm and tubine housing) only protrudes 2 to 2.5mm, this should align the flapper.