The manual page sleep(9) states for ltsleep:
   ...
   If the flag PCATCH is OR'ed into priority the pro-
   cess checks for posted signals before and after sleeping.
   ...
   If a ltsleep() returns as a result of a signal, the return value
   is ERESTART if the signal has the SA_RESTART property 
   (see sigaction(2)), and EINTR otherwise. 

What is does not tell is that with PCATCH the calling lwp might be 
stopped in ltsleep (to be exact in issignal called from ltsleep).
This can happen if:
	(1) The process is receiving a signal and the signal 
            is being traced
	(2) The process receives a signal that is not ignored, has
             the default signal handler installed and has the STOP
             property.

The problem with this is that processes might be stopped while holding
resources.
A good example for this are NFS interruptible-mounts.
A suspend (^Z) on a process waiting inside NFS can cause the process
to be stopped while holding a vnode lock.
Combined with the usual vnode lock crabbing this has the potential to lock
up the  entire system.

Maybe we need a PDONTSTOP? :-)

	Stephan