When you join a channel, it's log can be set to load in the channel window. As far as I'm aware in the current state this defaults to loading the last 500 lines of the log (it used to be more but got capped in mIRC 6.xx somewhere). It'd be nice to be able to adjust this number.
As an example, I frequently go back to a channel where I had a discussion just the day before to ctrl+f some link that was pasted, the channel might've been very active and the link doesn't show up in the channel window anymore because of the 500 lines cap. Now I have to go and open the log file, there's technically nothing wrong with that, but it just takes a lot longer (the process of going out and opening the log file, and the fact that they're usually multiple megabytes in size).
I hope this makes sense and can be incorporated in one of the future versions of mIRC. Or, if there's already a way to do it and I've just not found it yet, I apologize.