(let loop ([x 0]) (call/cc loop)) accumulates memory

Under Unix and Mac OS, a DNS lookup by tcp-connect blocks all
 MzScheme threads; note that the "net" collection contains a
 partial DNS client implementation that does not have this problem

Under Windows, if other OS-level processes write to or read from the
 same end of pipe as MzScheme, MzScheme may incorrectly report that
 the pipe is ready for reading/writing or it may prematurely read
 input (preventing the other process from receiving it)