"Last resort" does not mean "long process", it means, "only when there is no reasonable expectation of being able to improve fit to an acceptable level".
Whether it takes a while to reasonably determine that or not depends on what the problem that has manifested is and what opportunities there are to alter conditions to address the problem.
Internal processes only take as long as you make them. Correct the issue in 2 weeks. Not solved? Let them go. It doesn't have to be some 6 month ordeal of trying to get things worked out. The point is to have a process to deal with these things rather than telling the employee to pack up their shit and get out.
Besides, if they did let me go after a month, I'd have resented them for not giving me a fair chance.
How they didn't see it coming… Well, we informally discussed the project, my OOP knowledge etc… But they didn't read my blog, where my biases are quite clear. Come to think of it, my colleague didn't read the coding style rules he asked me to write either. If he had, some issues would have been addressed right away.
I was also told I would work on equal footing with my colleague, participate in technical decisions… He was my elder, and in the project from the very beginning, so he wasn't really my equal. But I failed to treat him like my boss, and it turned out to be such a big problem that the hierarchy made it official 10 weeks after my arrival.
My first commits weren't object oriented, so my colleague deduced I didn't know OOP. I lost all credibility at that point.
Finally, I was too careful. My unwillingness to rush the next feature as fast as possible without any regard for technical debt was interpreted as "doing research". Sorry, I just can't work that way. I was told we would "rewrite the code 50 times over", which would indeed have compensated. In practice we never rewrote anything. The first version always ended up being set in stone. Even code we both agreed was a mistake.
On the bright side, I did adjust over time, and they even said so (to me and my hierarchy). Maybe that's why they kept me for so long. But it wasn't enough to keep me in the middle of a general downsizing.