There was a quite nasty long standing bug in the x86-64 port.  The
interrupt gates had a DPL of 3, allowing user space to trigger any
interrupt.  I have not found a way to exploit it this to crash the
kernel, but it definitely shouldn't happen.  It could e.g.  cause
problems with drivers that do not handle shared interrupt properly.

This also broke some programs who assumed that int <random number>
causes a signal.