Test1.ino

#include <SPI.h>

void setup() {
  Serial.begin(115200);
  Serial.println("Test1");
  pinMode(MISO, OUTPUT);
  pinMode(SS, INPUT);
  pinMode(SCK, INPUT);
  pinMode(MOSI, INPUT);

  SPCR |= _BV(SPE);
  SPCR &= ~_BV(MSTR);
  SPI.attachInterrupt();
}

// SPI interrupt routine
ISR (SPI_STC_vect)
{
  Serial.println((char)SPDR);
  //SPI.transfer('X'); //Enabling this line produces problems
}  // end of interrupt routine SPI_STC_vect

void loop() {
} 

Test2.ino

#include <SPI.h>


void setup() {
  Serial.begin(115200);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }

  Serial.println("Test2");
  SPI.begin();
  //SPI.attachInterrupt(); //Enabling this line produces problems
}

// SPI interrupt routine
ISR (SPI_STC_vect)
{
  Serial.println((char)SPDR);
}

void loop() {
  SPI.transfer('A');
  delay(1000);
}