A quick search found this document on the internal RC oscillator calibration, which explains all you need to know: http://nic.vajn.icu/PDF/STMicro/ARM/STM32F0/STM32F0xx_intern...
It is recommended to stop all application activities before the calibration process, and to restart them after calling the calibration functions. Therefore, the application has to stop the communications, the ADC measurements and any other processes (except when using the ADC for the calibration, refer to Step 5. below). These processes normally use clock configurations that are different from those used in the calibration process. Otherwise, errors might be introduced in the application: errors while reading/sending frames, ADC reading errors since the sampling time has changed, and so on.