At high defoci you might start seeing some moire effects in 2D pictures (2D CTF and Defocus Map) caused by the fast oscillations in the corresponding CTF:
WHEN it starts to happen depends on your monitor settings (resolution). For 1024 by 786 pixel resolution this normally happens at defoci higher than -300nm. Disregard those or stay below -300nm.
REASON for this is over- or under-sampling: oscillations are finer then the monitor resolution.
If this happens the software sometimes warns you with a message in the status bar: