... I lost 1 mBTC once and was able to bruteforce it back because it was located on an address which was constructed by an algorithm.
It's comparable with HD wallets which generates all future addresses based on a seed. As long as you know the seed you are able to recover all the coins.
How do you brute force your coins back?
Since
https://github.com/gurnec/btcrecover and
https://github.com/magnumripper/JohnTheRipper could not do what I wanted I learned some python and made a script myself.
I still knew which algorithm I used to construct the private key so I just had to check al 70 million possibilities
My script was slow in the beginning, a whopping 7 keys/second/core! But I managed to speed it up to 2000 keys/second/core using stuff from
https://github.com/weex/addrgen.
With this speed I was able to retrieve my mBTC within a day. Hooray.
After that I let it run to see if others were as stupid as me.
There was. One. A used address with some similar construction to create the private key. There had only be 7 mBTC stored on that key, but another scriptkiddy probably took the money before me so that I didn't have to think what to do with it because it was an empty address again.
No way it was financially profitable to let the script run but I learnt a lot during the process.