Star Trek: Enterprise is being cancelled at the end of this season, so I suppose criticizing it at this point is literally like beating a dead horse. But I've never been one not to jump on someone when he or she is down, so...
This season, the show has gotten back to what it should have been about the whole time: The interactions between Earth and the various races (Vulcans, Andorians, Tellarites) who will end up forming the Federation. Of course, the producers got back to basics too late, when no one was watching anymore anyway.
And yet, even with this, they still manage to keep pissing on the Star Trek timeline, pissing off the few of us who have stuck with the show this long. The most recent cycle of episodes dealt with the Romulans trying to destabilize relations between the Andorians and Tellarites. That works. The Romulans were around back then. No one knew who they were or what they looked like, but they were around. The Federation fought a war with them in its early days, in fact, according to TOS episode "Balance of Terror." So far, so good.
The method by which the Romulans are doing this is with a special ship that can holographically appear to be some other ship. Thus, they can make it look like Tellarite ships are attacking Andorian ships and vice-versa. That's fine too. The only problem is that the Romulan ships in these recent episodes have warp drive.
Why does that matter? Well, you see, in the first appearance of the Romulans, the aforementioned TOS episode "Balance of Terror," the Romulans have invented the cloaking device but what they don't have is warp drive. Later, in a TOS episode called "The Enterprise Incident," we see that the Romulans are using modified Klingon D7 battlecruisers purchased from the Klingons. That is how the Romulans get warp drive: they buy it from the Klingons, more than a hundred years in the future of the recent cycle of Enterprise episodes. (The Klingons get cloaking technology in the exchange, which is why they don't have cloaking devices throughout TOS but then have them in the movies). The Romulans invented the cloaking device but never managed to invent warp drive themselves.
So, by giving the Romulans warp drive way back when, Enterprise has screwed up the whole history of the Romulans in the Star Trek timeline. But, then again, I suppose it doesn't really matter, because they screwed up the Romulan timeline completely in an earlier episode anyway, where a Romulan warbird cloaks in full sight of Enterprise, thus making the fact that Captain Kirk knows nothing of the Romulan cloaking device in "Balance of Terror" nonsensical.
I know that continuity is not the most important thing in a long-running series. Sometimes, in order to tell a good story, you futz around with things a little bit. But here, the producers chose to do a series set during the years prior to the founding of the Federation, so they intentionally put themselves in a situation where there is all kinds of preestablished continuity that they would have to keep in mind. And the fans are going to notice, because these aren't things that come up in a single episode then aren't mentioned again or something, but these are continuity issues that inform how the whole freakin' series progressed from TOS to TNG to DS9. As such, I have no sympathy. If you don't want to be constrained by continuity so much, don't decide to do a show set in the past of a well-established timeline. Jeesh.