I blew up the engine on my m3 on day 1 of ownership (big end)... oops. Thankfully had warranty.
Things can get expensive when anything goes wrong. I just had new clutch/flywheel and few related bits on the m3 which cost £1500 with me sourcing part for as cheap as possible.
The big inspection services are also costly.. almost £800 from an indy. Both my m3 and z4m just had those too :(
Awesome cars though and the appreciation on the z4 has covered all costs for the last 5 years