That was my experience way back when.
Most plastic pens would yield on a pants back pocket when you sit on them, sooner or later. On a shirt pocket, plastic pens with a breather hole in the cap, or with a snap-on cap would typically leak, again, sooner or later. For whatever reason, most plastic sections would break (develop a crack near the nib) too, sooner or later. If that was not the case, then the clip would break (plastic) or deform (metal).
Wearing them on a shirt pocket would often be safe unless I was to move too much, still, I would typically have a cloth, napkin, tissue or kleen-ex ready when uncapping for very often there would be leaks. I still carry or have one at hand always for this reason.
That is what set me on a quest to look for a pen I could wear on the back pocket of my jeans. Specially so I could wear T-shirts and sit carelessly without having to remember taking the pen out (at work, in the tube, bus..). I finally ended up with a Noblesse/Slimline that has lasted me close to three decades (and still goes on, but I have tried not to abuse it excessively --or may be I had learnt from previous experience).
More recently I discovered the Kaweco AL and brass Sports, which I have worn in my jeans pockets with (almost) no issue. The only problem is that the removable clip is normally not strongly attached, so if I use it to clip it to the pocket, sooner than soon, it ends up detaching itself and being lost. I no longer use those clips (though I keep them). I'm pondering whether to use the clip with the Kaweco Copper Liliput (because this is a lot tighter) but I've gotten used to just throw them in my front pocket. Now, I use jeans, they may not be comfortable/aesthetical without clip for other kinds of trousers/pants.
Sorry. Can't resist. I always remember Mae West's attributed line "Is that a gun in your pocket...?"
These last paragraphs seem to suggest that a metal EDC FP (of which there are many nowadays) may be the safe way to go for (pants) pocket pens.
Not so. I also got some of the latest DelikeAlpha/KawecoSport super-cheap clones. One of these last clones gave me a very bad surprise last week. Got it in my jeans pocket all day (I had gone out hiking, but previously had no issues with other mentioned pens) and when I reached for it, the cap had unscrewed and got free, and the nib had (from contact with the cap and keys) bent upwards. I've also had one of these clones where the plastic threading inside the cap has unglued and got loose (so the cap got free as well). I've been able to fix them, but learnt the lesson:
It is not enough to have a sturdy (e.g. metal) pen as an EDC, it must close well, that is, tightly enough that the cap cannot get loose. Consider that many metal pens (e.g. Jinhao 450 and 750) do have a plastic sleeve in the cap to clap it to the body, and that plastic sleeve is easier to damage. A screw system may be safer than a snap-on cap, but you also want to have enough threads (turns to uncap), a solidly fixed threading (better if properly carved in the metal as in the Kaweco), and proper ink flow control.
The last one was the only nice surprise in the "bad surprises" I mentioned: the cap had gotten loose but there was no relevant ink spill, likely because the nib was an EF.
So, to summarize: many pens, specially with broad nibs and snap-on caps will leak when shaken, typically inside the cap. If you hold them by the cap and shake them (as when they are fixed by the clip to a pocket), it is also likely that the cap will get loose and spills will leak out. This usually means not shaking with a pen in a pocket.
Not all pens are like this: sturdy pens will stand abuse, it is better if they have a screw-on cap (for snap-on caps tend to become loose with use, although well engineered ones, like Montblanc might still survive), preferably with several turns to close/open (3-4), if possible prefer a well engineered thread system (if it is too loose or badly fixed to the cap, the cap will open), and as a last resort, an EF nib is less likely to leak. An EF nib is also less likely to dry out (in my experience), which is good for screw-on cap pens.
Looks like an awful lot of things to look for but, thankfully, there are many affordable pens nowadays that you can get and use as reliable EDC pocket pens. The Kaweco Sport (plastic, AL, brass, steel, carbon fibre...) and Liliput, the OTOH Tasche, the Delike Alpha (although there are reports that it might be less reliable), and many, many more.
Edited by txomsy, 29 August 2019 - 08:15.