You know this is something that bugged me when i was messing about with guitar. It seemed to me that just fretting a chord does make a noticeable difference. Does this mean that guitarists should be tuning open strings a tiny bit flat in an ensemble with horns, maybe ??If you're very sensitive to this here's another answer. Guitars that are exactly in tune with open strings immediately go just a tad sharp simply by fretting any note / chord. If the guitarist uses a capo, same thing. The difference is very slight but present. If you're used to hanging on the flat side of a note, it becomes apparent as guitarists will always be just a tad on the sharp side.
Good point. It's probably because we're on a sax forum. The first rule when a wrong note is played is to look disgustedly at the guitarist.Everyone so far is pointing fingers at the guitar players. It is also possible that your horn ensembles are tuning flat.
Take out a tuner at either instance to see where the pitch is going.
Sometimes a lil' awareness drill can be interesting - it can get everyone to participate in active listening.