Jacked up X5

[SGX45]

I have a 04 X5 with 20k miles. The coolant light went on and I took in the dealer. They said they will have it back by 4. I called after 4, and they said after further investigation they found oil in my coolant. They said it might be a blown head gasket and they will need a few days to look into it further.

How does something like this happen on a 20k mile 55K $ Suv? Is something like this common?

Not too long ago they had to replace my thermostat housing.

Anyone have similar experiences or am I the only one with problems?

I sure hope I don't have a lemon.

Thanks.