Literally minutes after the race began ... a fan holding a cardboard sign held it out too far ... hitting one of the riders ... causing him to eat it hard. That, in turn, triggered a chain reaction as rider after rider bit the dust in a wild scene.

Here's how it was described by former green jersey winner Sean Kelly ... “The guys at the front went through a very tight section between the islands in the middle of the road and it was a dangerous one for those further back in the peloton. This is Brittany – you have a lot of road furniture and small villages. It is very technical for the first stage of the Tour de France.”

As if that's bad enough ... there's a real chance of rain as the day wears on. Believe in bad omens much?

It's unclear if any of the riders were injured, but gotta say ... if none were, it's a minor miracle.