"There's nothing that a liar hates more than finding that another liar has lied to them"

from "A Red Herring Without Mustard" by Alan Bradley

When the speaker has a mental reservation about a statement they are making, (a lie) they may cross their fingers behind their back (so the hearer doesn't see) 
The gesture is from early christian times, or perhaps earlier.  It is said to be reminiscent of the cross and meant to ward off retribution for the lie.