One bit of discussion in Nathaniel Torson’s Time Traveller’s Companion chat was about TARDIS systems, what the Chameleon Circuit might get stuck on and the like.
But of course there are other systems that could also go wrong, temporarily or semi-permanently. This is covered in the TTC in detail, but here’s a random selection of adventure-friendly inconveniences...
Getting stuck in one shape is annoying if you want to blend in, but there are other ways the disguise system could go wrong. It could become something like a tyre - small and mobile and prone to rolling off. It could become inconveniently big, or inconveniently small - where do you climb into a statue? And of course, it could disguise itself too well, so you need the beeper on the TARDIS key to find it.
The TARDIS doesn’t translate for the travellers.
Consider the issues of having to learn a language in almost every adventure. Pick up a “universal translator” gadget that isn’t all that reliable, or count on a psychic PC to translate accurately, or...
This happened temporarily in an adventure during our group’s playtest - in which two of the four PCs could understand each other and no-one could understand the TARDIS systems.
The TARDIS routinely materialises in thin air about ten feet above the ground and then hovers there. Buy a ladder.
The TARDIS routinely materialises on its side.
The TARDIS routinely materialises with its door on top. Buy a ladder again, this time to climb out of it.
The TARDIS never quite comes through.
Sometimes, neither do you. You didn’t really need to be able to pick things up during this visit, did you? Eh?
The TARDIS needs to go into the Vortex to teleport or travel in time. Without that, it’s still a pretty fast spaceship, but...
The TARDIS could drift out of phase, relocate itself closer to the travellers (without telling them) or even appear around them without being called to when it detects danger. A bit of an inconvenience when you’re trying to negotiate with a bunch of angry Sontarans and reassure them you’re unarmed...
The TARDIS external shell is exactly as tough as the thing it appears to be.
Or, conversely, the defences extend substantially further than they normally should, and anyone without a key is bounced away if they get within ten feet of the shell.
The “Bigger On The Inside” interface
The TARDIS isn’t bigger on the inside any more.
Imagine if this one failed. It would settle the question of the size of the TARDIS once and for all, and make parking it a right sod. (The other way, as seen in Father’s Day, shows just how small a police box actually is.
Of course, a different TARDIS might have a lot less rooms than the Doctor’s, so it might be less of an issue - like parking a vehicle the size of a Georgian townhouse rather than, say, the Olympic Stadium. Really old ones are the same size inside as out, and really new ones might have cleverer interface tricks, allowing everything to be stored in one changeable room...
The TARDIS does that to an extent, but how much...?
Of course, if that goes wrong, you might be stuck with the door to the outside world being at the bottom of the swimming pool, or the console being on the outside of the ship, or...