Bad mom

Okay, my son is 6 weeks old. We went to the dollar store today and his car seat wouldn't fit in the basket part of the buggy so I put him on the top where most people place the car seats (this is the first time I've done this.) Everything was going smoothly until we went to walk out the store. The door hit the buggy and his car seat fell off the top. I caught him before he hit the ground and he was strapped in properly.  He was sleeping and continued sleeping through everything. I feel like such a bad mom. This is the first time anything like this has happened. There were other people outside that checked on him and comforted me through this traumatic experience and told me things like this happen to everybody. I feel so horrible. Do things like this really happen to everybody?