If Serial0 is used for other than debug, the output on Serial0 that is created during start can create problems. Example output:

ets Jun  8 2016 00:22:57

rst:0x1 (POWERON_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:1
load:0x3fff0010,len:4
load:0x3fff0014,len:732
load:0x40078000,len:0
load:0x40078000,len:11572
entry 0x40078a14

### This output can be suppressed by pulling GPIO15 low.