Not sure what you mean. I'm guessing you mean when someone lies about nothing having happened when something did or who "lied by omission" by skipping over dirty laundry, past abuse, etc. Not a problem for me (though I'd rather they just say they don't want to talk about it, I usually don't take that personally and pretty much assume they'll talk about it when they're ready, and if not there will be other things to talk about instead).
I'd be more disturbed if it was seeing someone behind my back. Mainly I'd be worried if it was a stranger and could be angry for his or her own safety. And my BFF used to be the BFF of my past partner and I said she didn't have to choose between us (whereas my ex did tell her to choose) and if they started getting close again without telling me I'd be hurt (because I wasn't told).
Now obviously if this is hiding something they know would get them in trouble (that is, they're trying to protect themselves from consequences of having broken an agreement, rule, etc) then that's a different matter. Offhand, my BFF wasn't able to pay me back (and I believe her) but if I found out she was able but wanted to keep hold of it just in case she lost her job then I'd be very angry and hurt (though less so if she fessed up than if I busted her for it). I expect I'd forgive, but they wouldn't get away with it. Of course if it happened repeatedly then that would be another matter and I'd distance myself from them at the very least.
If the lie were to cause harm to someone else (like shifting blame to protect one's self) then that would be very serious to me.