Which Brokers in the market manipulate Price to their advantage?

Currently developed trading robots for testing and used a few brokers trading platforms to trade. Found something fishy but need some echos here to point out which brokers maniuplate price to their advantage.

e.g.
1) which brokers do not allowed scalping robot?
2) which borkers used magic number or order comment written by client to identify the trades in which they can 'play" priority/privilege/time delay/spread tricks to their advantage?
3) any more here?........

thanks for the thread.
I have a blacklist of a few major brokers.
e.g.
1) their response time were significant slower once the account balance is low( could be no time to serve low account holder unless you top up more $). I ran the same platform via 2 different internet access, same time, same trading command fired, but the one with lower amount in balance was served with lagging , obvious lagging. other commands also have this 'symptom'. Then I opened a few more accounts, including live account, and ran it til it was lower than about $100, the lagging or non-response time were both significant. it could be up to 30 sec to 1 min. Brokers claimed $25 can open an account, but I don't think traders could get the benefit of low initial deposit players.
2) some brokers run the contest but the results they annouced every day does not match the final result. anyway, they have protection on the games rules they set.
3) having 2 internet access lines(same ISP and speed) could definitely bait the brokers to bite the EA or not. I suggest anyone who is interested to compare , and if your EA trading strategies can also be operated manually, may run different test on their platforms or mt4.