I am a TA for an undergraduate class, and I have the following situation:
Usually, I give my (one-hour) lectures (which consists of solving exercises and reviewing theory as needed) at Thursdays 12 p.m., but this week they have the midterm right at this time, so I decided to move this week's lecture to Tuesday (today) to help the students. The professor is aware of this and saw no problem, although it might be relevant to say that I had no obligation to do so: I could say "there's no lecture this week" and be done with it. Instead, I said that this lecture would be at the same time, 12 p.m., but I mistakingly wrote 1 p.m. at the webpage of the course. When a student emailed me yesterday asking for confirmation, I hastily confirmed the 1 p.m. mistake, only to realize the screw up.
To minimize the harm, I decided to show up at 12 p.m. and give a two-hour lecture. I put a follow-up message on the webpage informing that I would be there earlier, and sent an email to that student (I do not have everyone's email address). A lot of people showed up at 12, asked their questions, then a lot more people appeared at 1, visibly upset, and asked their questions. All in all, in the end, it seemed that everyone had their questions addressed, so I won't lose any sleep over this.
I would like to know, though, if I could have dealt with the situation better. Or if there is a standard strategy for dealing with this kind of mistake (certainly I'm not the first nor will be the last TA to do this).