Short circuit protection stage in the 741 op. amp
If you look at the circuit diagram of a 741 



you will see this at the top of the output stage...



The output path is through Q14 and R6. When the current though R6 is high enough it turns on Q15 which starts to turn off Q14. This limits the amount of current it can ever source to about ~0.6V/27R = ~20mA.

It is your basic current limiter circuit.

A similar circuit, PNP version, exits below that to limit the sink current.