  Our MAMBO 1.2 mm blank field imaging survey of ~0.75 sqd has uncovered four
unusually bright sources, with flux densities between 10 and 90 mJy, all
located in the Abell 2125 field. The three brightest are flat spectrum radio
sources with bright optical and X-ray counterparts. Their mm and radio flux
densities are variable on timescales of months. Their X-ray luminosities
classify them as quasars. The faintest of the four mm bright sources appears to
be a bright, radio-quiet starburst at z~3, similar to the sources seen at lower
flux densities in the MAMBO and SCUBA surveys. It may also host a mildly
obscured AGN of quasar-like X-ray luminosity. The three non-thermal mm sources
imply an areal density of flat spectrum radio sources higher by at least 7
compared with that expected from an extrapolation of the lower frequency radio
number counts.
