There is a street light I pass frequently that occasionally flashes like a strobe.

I've seen a bright flash at that corner a number of times over the past year or two, but never saw the source.
Usually there's a building blocking it and all I see is the street light up.

This past week I was coming from the other direction and clearly saw the street light flash twice, very brightly.

These lights are bluish white, I've always assumed they were mercury vapor, but don't really know. It's not a city
likely to have moved to LED.

Is there a failure mode for a discharge lamp that would produce that?

High-pressure sodium lights exhibit a "cycling" on-and-off behavior when
they reach end of life. However, when I've seen this it's been a
fairly slow cycle, and didn't exhibit the sort of bright strobe-like
flash you describe.

I suppose that you could be seeing a mercury-vapor lamp with a failing
bulb and/or ballast, where the ballast sends a strong pulse to
"strike" the lamp and get the arc flowing (hence the flash) and a
fault in the bulb causes an over-current or under-current condition
which then shuts down the arc almost immediately.

Pragmatically, you might want to write down the streetlamp pole number
(if it has one) and contact the city's public-works department and
report the problem.