This means premature failure in impact applications.
Blue Brittleness
Upon heating some plain carbon and alloy steels-not necessarily those that have been quenched to martensite or bainite-can exhibit both an increase in strength and a substantial decrease in ductility/ impact strength.
In this low temperature range, we call this effect blue brittleness. 
Blue Brittleness is a strain aging mechanism that occurs in this blue heat  temperature range.

Link to graph.
In the range of 700 to 1070 degrees F ( 375 to 575 degress C) most common low alloy  steels show an increase in their ductile to brittle transition temperatures- regardless of whether they are heated into this range, or slowly cooled through it.
(Think large section parts/weldments).
Lower Manganese (below 0.30 wt %) containing plain carbon steel grades do not seem to be susceptible, although elevated levels of Tin or Phosphorus can make even these grades somewhat suceptible.
For these reasons, we would NOT use any tempering cycle below 1100 degrees F (~595 degrees C) for the common carbon and alloy constructional steels typically designated by AISI and SAE in North America.
This gets us past the ductility troughs seen on the above figure.
500 Degree F Embrittlement
500 degree F Embrittlement also occurs in quenched and tempered High Strength Low Alloy  (HSLA) steels  when they are subjected to a temperature range between 400- 700 degrees F (~ 200-370 degrees C). This differs from Blue Brittleness in that it is a phenomenon of tempered martensite, it is not related to strain aging. I was taught that it is rather a result of  precipitation along prior austenitic grain boundaries.
 Proper selection of steel chemistry is the best defense against this type of embrittlement, with Aluminum additions above 0.1 weight %  usually effective at preventing the problem. (Some steel producers lack the ability to add  Aluminum to their steel melt due to technology constraints on their casters…)
400 to 500 Degree C Embrittlement
If the steel is high Chromium content  (15% or more by weight) it can be subject to embrittlement when held in a 400-500 degree C  (~750 -930 Degrees F) range for a long enough time.
(Think heat affected zone in welding Stainless steels.)
This embrittlement can be eliminated by a proper soak at a higher temperature to redissolve the carbide (and possibly nitride?) precipitates.
Conclusion
These are the primary forms of embrittlement that I have encountered in my career. Other types of embrittlement  can include Liquid Metal Embrittlement, Sigma Phase Embrittlement and Embrittlement driven by neutron irradiation, or environmental factors such as hydrogen absorption, (often in plating) or Stress Corrosion Cracking where outside chemical attack and mechanical stress produce fine cracks in the steel.
Bottom line: Thermal treatments, and post quench temper treatments below 1100 degrees F are not recommended because of their possible embrittling effects on susceptible steel grades in common use in North America.

 

  With increasing temper temperature, the plasticity of a quenched martensitic structure increases up to around 400 degrees F; decreases to a minimum in the 450-700 degree F  (230-370 degrees C) range, and then continues to increase.