I can, twice
First time: Through welder's glasses during the March 1970 total solar eclipse.
Second time: During the winter of 2001/2, near greatest western elongation, on my way to work in the pre-dawn hours. Confirmed later that day by "Stardate" (the U. Texas radio program, syndicated on many NPR affiliates).
The other times I've seen Mercury, it was deliberate. That includes during a local star party, where I looked at it through a hand-built 8" Dobsonian. No surface features; phase only.
That's really cool, Jacob -- glad you were treated to one.