I'm struggling to imagine a scenario where dadoing <1mm from the fence was a good idea in the first place. I'm assuming you're talking about a miter/crosscut fence but still...
Cutting a rabbet? Probably should have used a sacrificial fence, but it's easy to see how this could happen.
I had a sacrificial fence (wood) on the aluminum fence to reduce blowout. I had the aluminum extend to near the blade to reduce the deflection of the whole thing. It was too close.
Really, I should have sacrificed a crosscut sled for dado usage, but it really chews them up.
Could be an aluminum extrusion or something in a miter gauge or cross-cut sled.
I've stopped myself just in time from triggering my Sawstop while using an aluminum miter gauge.
The scenario is that the gauge was set with reasonable (1cm) clearance at one angle, and then I changed the angle without re-checking the clearance, and the back side of the fence swiveled into the blade path. Pythagorean fail.
Or, perhaps with GP comment, used a setup with reasonable clearance for a regular blade, and then put in a wider dado, which ate into the clearance.