When you attempt to make a post dated prior to an existing entry without the backdate option turned on, most clients (including web) give you an error explaining this. However, iJournal (1.54, under OS X 10.3.2) gives a generic "error posting, please try again in a few minutes" message. I didn't find a bug in the tracker related to this, but I'm not very good with the tracker--so if it's already in there, don't mind me; if it isn't, could someone please add it?
(This is difficult to reproduce, as the recently-mentioned bug about "Use current time" has reared its head, turning on "Use current time" when I post after unchecking it.)
(Well, except for this post, when I wanted to use the current time and it didn't recheck it when I forgot to.)