What mistakes to Arminians make? What are your objections to Arminianism?
The way in which Arminianism makes similar mistakes is by noticing some of the Scriptures Calvinists seem to dismiss and accepting those, but then also dismissing Scriptures which become inconvenient for the theology. This results in the same kind of net effect: A systematic theology that works on paper, but just isn't biblical.
This is understandable, the temptation to want to synthesize a coherent system of theology, where the whole and its parts all make sense together is strong. It's what we want to do as human beings with everything--we like doing this, to categorize, to catalogue, to systematize, synthesize, to take many disparate ideas and make them a cohesive whole that we can then wrap our minds around and digest and explain easily.
The problem is that when we look at the Bible, we actually come across ideas that really don't want to work together--they are irregular shaped pieces that don't want to fit into the nice, tidy, little holes we've carved out. So what is one to do with irregular shaped pieces that we don't know what to do with? Well the easiest thing to do is to ignore them, and if we can't ignore them, to maybe chisel at them and then force them into the holes we've made.
This is why, at least from a Lutheran POV, both Calvinism and Arminianism are getting things significantly wrong: they end up with irregular pieces that they either need to ignore or try to force into the system. The Lutheran approach is to say, "Yes, those are irregular pieces, and we don't know what to do with them, but we will believe them as they stand, even if they seem to conflict and disagree with these other pieces over here." That is, when Scripture says A and also says B, and we have no idea how to make A and B work together, we can accept that we just don't know how A and B work together, and won't try and make them work together. If that makes it sound like we contradict ourselves, that is fine.
It's not about having a systematic theology that all makes sense by its own internal consistency.
-CryptoLutheran