Soft lithographic microcontact printing using the residual polvdimethylsiloxane (PDMS) found in elastomeric PDMS stamps is demonstrated to lead to unstable prints with sub-micrometer dimensions. The statics and dynamics of the process have been followed with time-resolved atomic force microscopy, imaging ellipsometry, water contact angle measurement, and optical diffraction. It is proposed that this instability places a fundamental limitation on patterning by macromolecular fluids, which is of general relevance to soft lithography and nanoimprint lithography with low viscosity polymers.