Relatively early on I built a portal to the nether, made it "safe" by building a fully enclosed stone hut around it with an iron door and obsidian pressure plates. I left the overworld side "open to the weather" or as I should say "open to all of the mobs using it as a safe-house during the day".
The other night I decided to check on my lava pump, farm some blazes and mine some nether quartz. I went through the portal and had just enough time to see a face full of creeper. It took multiple zerg runs to finally kill everything that was in that "safe" room. Lost all of my wand foci, top-tier TiC tools, Flux Infused Sword, and a raft of other stuff.