This happened with me once...
It really tried to put me off raiding. My recon was standing besides a ding-dong which she didn't detect, but was outside the blast radius.
I felt that I was safe from traps, so I moved all of my survivors towards the enemy fortress, until they all met up at the same location. What happens next?
1 - Recon moves one step, detonates ding-dong
2 - All 3 Recon survivors get downed
3 - Leader gets blown towards another ding-dong, the end result is predictable
4 - Engineer flies towards scatterbomb, another explosion, flies towards wire trap, downed