Since end of the Beta (if I remember correctly) mobs glitch through walls and fences. It's really annoying, especially with hostile mobs and the bug is still present in Minecraft 1.2.5. Someone of the bukkit peeps should tell it to Mojang as they don't seem to know that bug there.
I watched that for months and no-one cared about that bug ... it's just annoying and I don't understand why it won't be fixed.
There is a Client/Server "Fix" http://www.minecraftforum.net/topic...-almost-fix-for-mobs-glitching-through-walls/
I tested with Bukkit #2156 and with/without BetterSMP Client. It isn't fixed at all. I just wonder because this bug first occured somewhen during the Beta. AFAIK this bug didn't exist in Beta 1.2 for example. So it worked long before.