Several people have mentioned aikido changing people for the worse
. While I think that the nature of aikido usually nudges people in the right direction, I too have seen negative change happen. (its possible to get bogged down in a relativist swamp re what is "positive" or "negative", perhaps healthy and unhealthy is a better model.
The most common kind of unhealthy pattern one see is ego INFLATION
For instructors in particular this seems to be a problem. One very senior UK Sensei warned me, "Don't believe your own advertising."
I routinely see the same thing happening with colleagues who work with children. There are several definite phrases that outdoor sports instructors go through in about a year. These could be labeled:
Innocent Hero (semi capable, loves kids)
Ego freak (thinks fully capable, loves being loved)
Cynic (capable but closed, loves kids)
Open again (loves kids, learning again)
Not everyone makes it through the whole process
of course, and it is just that - a process, not purely negative
Perhaps aikido is particularly prone to ego issues as there is no winning and hence no loosing. The dojo oasis has few reality checks.
The ego problem is essentially a symptom of a wider issue, that of aikidoka wrestling with power issues. I've observed that in general aikido balances people's personalities, but that sometime the process can go badly wrong. For example, sometimes the experience of vulnerability in aikido can make people more compassionate, but can lead to an unhealthy desire never to be dominated. Similarly some start to get a kick out of the feeling of power over their dojo mates, and need to be reminded of th O'Sensei quote, "Aikido is not for correcting others, it is for correcting oneself."
Incidentally, before any of my friends post - I have a big fat ego and I know it