I find this assetion also puzzling. If the source was a level trigerred interrupt the int flag is set and cleared by the dispatch to the isr - so I doubt that could be ignored. The only way I could see this happening is the higher priority isr resetting the lower priority int flags - thus the lower priority interrupt would be effectively ignored.

Where possible I try to minimise the number of interrupt sources to keep the stack as small as possible and also to minimise the possiblity of weird interactions that may happen due to coincidence of different interrupts - obviously careful coding is required.