SRI-NOSC/doc/tty.chars.lost

One problem that shows up in the asnchronous tty driver is that characters
are occasionally lost when both Unix and the user are typing.  Sometimes
just an output character is dropped, more frequently just an input character,
most frequently one of each.  This happens across a wide range of PDP-11
devices, so the problem is not with an individual driver.

It also happens with the synchronous tty driver if you try to type ahead,
but since this is so unaesthetic, it is rarely done, and hence is not a
real problem.

I suspect that somewhere in tty.c is a buried assumption that a tty is a
half-duplex terminal; that is, that input and output operations will never
be going on simultaneously, so that at some point where it should lock out
interrupts, it doesn't.  I've never been able to find it.