The swordsman (speaking from experience)...
...assuming he's wearing plate, there's lots of subtle ways of messing up his armour.
Arming jacket (the undergarment to which the armour is strapped):
The most obvious would be to sabotage
the points... the waxed cords tying the armour to the jacket. Most armour is also buckled on, so losing a point doesn't just expose a target area, it also greatly hampers the wearer. For example, I once lost the point on my spaulder (a trilobite like shoulder and upper-arm protector). It fell free of my shoulder, but remained strapped to my upper arm armour, rattling about and pulling my cuts off balance.
Slightly more devious would be to cut the stitching on the
voiders, that is the mail patches sewn to the arming jacket to cover gapes in the plate - typically throat, armpit and crook of elbow. They would gradually come undone during the fight, not only leaving dangerous gaps, but also hampering the wearer.
A really really devious saboteur might play with the
fitting.
A little extra padding on the chest, and the breastplate hampers the sword swing (the edges dig into the upper arm as you cut), on the belly, and breathing becomes harder, on the arms, and the armour cuts off the circulation if you tighten the buckles to the usual hole in the strap.
Remove some padding from the shoulders, and the cuirass chafes like crazy, bruising the collar bone.
Yet more evil, change the position of the points. Now the armour, subtly, does not work.
Finally, re-sew the voiders so as to hamper movement, e.g. too tight between arm and chest.
The Armour
Messing this up is harder, because some problems would be obvious as soon as you put it on. However, this could mean having a choice between conceding a fight, or going in with 2nd best kit.
If the
helmet has integral padding, try removing some of the stuffing. This will misalign the sights (eye holes), but also - for some kinds of helmets - cause the helm to grate against the shoulder defences, making it hard to turn your head properly. For a delayed action effect, replace some of the padding with something more crushable.
You can also go after the
straps. The caveat is that a good squire should notice if a strap is worn our partially cut, and that in the process of putting on the armour, most straps undergo considerable strain - if they're going to snap or come loose, that would be the moment.
More subtle would be to partially saw through one of the
sliding rivets in articulated defence. After a few rounds, the rivets would snap, leaving a gap in the plate and a hampering bit of metal flapping about.
Finally, most
gauntlets are riveted to leather which is in turn stitched to an inner glove. Snipping the stitches could cause the gauntlet to come apart at a crucial moment.
The Sword
There are practical problems with getting hold of a knight's sword, but if you can, a few minutes in a forge can temper or detemper the blade, meaning that it will shatter or bend at odd moments.
* * *
In all of the above, remember that a knight's panoply was maintained like a Formula One car. A good squire should notice any obvious problems, and just arming up puts the various straps and rivets under strain and reveals any obvious problems.
