If I use RS232 communication on a system with -at least- one timer, one
of the interrupts (timer or serial) will be masked during other
interrupt routine based on the interrupt priorities. This means that
I can loose a timer interrupt or an wait forever for the TI interrupt.

Making timer interrupt priority higher than the serial interrupt, and
designing a robust protocol which handles data lost and timeouts may
solve the above problem, but what if we have several interrupt sources?

Say we have serial interrupt, external interrupt and timer interrupt(s).
Is there a way not to loose the interrupts? Can we keep other interrupts
on pending state during an interrupt routine?