Character mistake: Sometimes in the show, when a Goa'uld or Tok'ra is speaking (as opposed to the host speaking), they will say "my symbiote", such as when facing danger or illness: "my symbiote will protect me" or "my symbiote will heal me." However the Goa'uld and Tok'ra are the symbiote and they should be referring to themselves protecting or healing the host's body.
Factual error: Before SG-1 switched to P90 PDW's, they carried MP5 submachine guns. They were often shown with a full magazine taped upside down to the one in the weapon, so that to reload, all they have to do is remove the magazine, turn it over and insert the full one. Team members were also shown doing just that in several fight scenes. Problem is, real special ops troops are specifically taught in training never to do this. It's far too easy to damage the lips on the spare magazine, rendering it useless. There are clips that will hold magazines upright side by side, but its easy to see on screen they weren't using them.
Plot hole: Anytime an SG member starts acting weird or something happens that is "impossible", SGC personnel always act skeptical about it. Considering what they have seen in the course of the show, "that's impossible" should have long ago fallen out of their vocabulary. And while I realize that Sam and Daniel are scientists, they have seen enough examples of incredibly advanced technology that they should be far more open-minded when looking for explanations.
Other mistake: In several episodes throughout the series, SG-1 is coming through a planet's Stargate but there's no sign of a MALP, even when it should be in the shot. SGC wouldn't send a team through without sending a MALP first (not only to check for a DHD, but survey the land, check for habitability, etc). And if the idea is "well, maybe they have a recovery unit go fetch it before SG-1 goes through", then why would the MALP be present in all the other episodes?