Inconsistency in Immortal tingle/sensation
A few examples of these oddities are, in the season 2 episode "Under Color of Authority" where Mako is able to enter Richie's apartment and catch him naked in the shower (poor Richie, he looked like he was almost going to cry for a moment) before Richie's immortal sense kicked in.. later in the episode Richie is at a motel and senses the same guy and even though Richie looks right at him out the window, Mako doesn't seem to sense Richie until he comes up the stairs and is near the room.
Another example is in the episode "Legacy" where Duncan and Amanda are meeting with Rebecca's husband at her gravestone.. Duncan senses an immortal, Amanda does not and thinks he's imagining things.
I think it's a good thing, because it makes it interesting if the Immortal sense/tingle can sometimes work from a block away or when they are both in the same room or seemingly dependent on who is paying more attention or is perhaps focusing themselves or something which would end up giving one immortal the element of surprise.