Despite popular belief, Mycroft was not, in fact, omniscient. He did, on occasion, miss things. Usually, they weren't big things. They weren't important, or he he could account for the missing information through other means. For once, though, that was not the case. In fact, considering the arrow sticking out of his shoulder, to say it was not the case was most likely an understatement!
Mycroft could still die. It was difficult, but it was possible. Over the many years, he'd had a few close scrapes, accumulated knowledge giving him an unfair advantage which ultimately helped him to come out on top. Most of those moments had been high pressure and dramatic, though. This? This was embarrassing. if Mycroft was to die, he did not want it to happen at the hands of an unexpected arrow!
If it weren't for the initial shock of the assault, he'd have been muttering curses to himself about the audacity of the situation. As it was, though, Mycroft was silent as Arthur caught him and pulled him behind cover, momentarily deadweight before, a few seconds later, managing to regain control of himself.
As he motioned to his shoulder with an uncharacteristically shaky hand, the arrow still embedded in the flesh there, Mycroft also looked to it to assess the damage. It was hard for him to see it properly, but he was certain it didn't look good. Whether it would kill him remained to be seen. It didn't seem like irreparable damage, but wounds could so easily become infected, and the archer was still out there.
"Don't pull it out," Mycroft warned Arthur, his hand moving to grab the other man's wrist just in case he decided to ignore the instruction. "It's too risky here, and we don't have the correct tools." Nor did they have anybody suitable for the job. Mycroft wasn't going to be up to it himself - it would be too hard to position himself correctly, even without the lingering tremor still on display - and while Arthur was a man of many talents, arrow removal was not one of them.
"Which of us do you believe was the target? It seems more likely that it was you, but we cannot rule out the possibility that the arrow hit its true mark."