It can, in fact, be quite a big deal.

Was it the "Mixes With Anything" type, or just regular old anti-freeze.

The "mixes with anything" stuff is made up of:
http://www.mightyautoparts.com/pdf/msds/vs7/af-888.pdf

Ethylene Glycol
Diethylene Glycol
2-Ethyl Hexanoic Acid, Sodium Salt (Corrosion inhibitor)
Neodecanoic Acid, Sodium Salt (Corrosion inhibitor)

It is both Nitrate Free & Phosphate Free, and claims to be OK with ANY other coolant chemistry.

Other coolants use additive packages that may not be compatible with the BMW OEM coolant.

Having read up on this when I was a VW owner, I know that there are certain combinations of VW OEM coolant (G12, G12+) with other coolants that could have catastrophic consequences.