If it is severing the R wire in response to a limit situation, that means the furnace's wiring is not compatible with the Nest.
The furnace is designed to have a simple mercury make-or-break 'stat. Interlocks are also make-or-break, so your furnace has simply placed them all in series without caring about sequence. As a result, some of those interlocks are interrupting the R line when they should be interrupting the W line.
Now, if your system also had air conditioning or auxiliary heat stages, they would also be shut off by an interlock that severs the R line. Study the system carefully to see if that is intentional/desired. If it is not, the furnace should be rewired so those R-line interlocks instead interrupt the W line, and nothing interrupts R. That will make it compatible with the Nest.
If C is being interrupted, them you got C from the wrong place.