I think I've identified it.

In the here and now, it's generally considered a bad idea for people in supervisory positions to have romantic relationships with their subordinates. Yet here, we have the following:

Gryphon got home a little after six, for a change, and spent the
first few minutes after his arrival wondering just what he was going to
do with himself. With the younger crowd out marauding and the older
ones either on assignment or off doing their own stuff, he was at a bit
of a loose end. He considered finding out what Paige was up to and, if
it happened to be work-related, tagging along, but -
"Evening, Chief!" said a voice cheerily from the archway into
the living room. "Got a minute?"
Gryphon whirled, yanking a plasma pistol from inside his coat,
and found himself leveling his weapon at a grinning Sara Sidle, who
stood leaning against the archway, arms folded.
"Gah!" Gryphon blurted. "Wh - how did you get in here?"
Sara looked back over her shoulder, then said nonchalantly, "Oh,
uh - turns out I can run through walls if I try hard enough."
"Jeez." Gryphon put away his weapon. "Don't do that. I
could've shot you."
Sara gave that a moment's consideration, then said matter-of-
factly, "Actually, I don't think you could."
"... This is going to be a problem, isn't it," Gryphon observed.
"That you can't shoot me?" Sara asked with a quirky smile.
"That your attitude's getting weird," Gryphon replied.
"Oh, come on," said Sara. "You're always after me to lighten up
a little."
"Aren't you supposed to be at work?"
"I took the day off," said Sara casually, as if that were
something she, well, -ever- did. "Got plans for dinner? I thought
maybe you could get a jet or something and I'd race you to Perth."
Gryphon eyed her skeptically for a moment, then grinned.
"You're on. Just give me time to get to Mathews." As they left the
house, he asked, "By the way, just out of curiosity, does this mean
we're Sort Of Dating Or Something again?"
"Well," said Sara judiciously, "Catherine said you were worried
about me getting hurt by your enemies? And I'd like to see them try to
-catch- me now."
"Don't get cocky," Gryphon cautioned her.
"What, you don't -want- another shot?" she asked, breaking out
her quirky smile again. Then, shrugging with affected nonchalance, she
added, "That's cool too."
"Ahh, I didn't say that," Gryphon replied.

If he followed through on that, I think that would be the mistake right there.

-Joe-