Przeszłem, przeszedłem

Often, without even thinking about it, I say przeszłem instead of przeszedłem, wziąść instead of wziąć. And this isn't a typical mistake for an Englishman, Poles do it too, although not as often as I do :D

Where does this annoying mistake come from ? I often mix endings with similar words and people often correct me, and it's getting increasingly annoying. Are there any other similar sort of mistakes you make ? I do hope I'm not alone :D

The same place that the Poles' saying it comes from - the general tendency for people to regularize irregular forms and to prefer shorter over longer forms. Basic linguistic principles.

I used to have a tendency to overregularize -ami and would say things like przyajacielami (instead of przyjaciółmi) when i asked native speakers why they never corrected such forms they said "eh, I understood, and some Polish people say things like that too".

I'm hopeless with numbers and mostly make no real effort to decline them and no Pole has ever mentioned it as a problem.

Also I'm not good with subtler questions of register (language variety associated with life role). A friend was once horrified to hear me say 'na razie' to a colleague who outranked me so I stopped saying it to higher ups.

HAHAHA, hierarchy problems :D
Poles do have something with that, it's as if some people are higher ranked in society than others, and should be listened to, even though there is no particular reason for it. I often also get a "foreign pack" impression - I behave normally, put on a smiley face and generally speak in a very friendly way to everyone I meet, whilst the person I'm speaking to often looks bemused by my behaviour, and seems to ignore most of my gestures and comments, and sticks to the things that matter in the conversation in a very strict way. In England you don't get this, you can speak openly even to strangers if you're lucky, but in Poland you've got to stick to the pack, if you know what I mean, you can only speak like this to people you know.

It's more or less the same with "poszłem" vs. "poszedłem". The former is still "Polish", only it's pure slang, whereas the latter is "grammatically correct", i.e. proper Polish:-)