There are a number of errors that fall under this category. But they all have the following common elements:
If you find your EM35x in this state, there is a general recovery mechanism. You will need to short the nBOOTMODE pin (PA5) to ground, execute an em3xx_load command (such as '--erase'), then release the PA5 pin. After this procedure, the EM35x part should function as normal.