finally I realized where some of the odd price jumps come from: it's people confusing buy and sell.
I don't remember what it was like with the old site but with the reloaded one it is very easy to buy instead of sell if you forget to click the sell button.
Depending on how much fiat you have in your account and how high you set your order (the higher the more conservative you think) you eat well into the asks.
Of course this can be fun for bystanders but it is less fun if it happens to you (it almost happened to me and I almost got a heart attack).
I am pretty sure people have already trained their bots on this and also it may be profitable for MtGox in the short run. In the long run this unnecessary volatility harms Bitcoin, though.
There is a very simple solution:
MtGox, please change your UI so that you have to click on either buy / sell before you can enter any data.