Maybe surprisingly, not often. You were used to the systems you had & you just got on with it. That's how it was. Mis-indents could occur , but I didn't witness many of them. PRI only was good experience. Later, we all got used to SSR & Code Callsign Conversion but it was revealing to see that those controllers who didn't have PRI only experience found it very difficult, during TRUCE PRI only exercises, to continue to provide radar service without SSR; & even without CCC.