One of the things I've found in a number of Cessna taildraggers is mis aligned landing gear. Gross mis alignment becomes FAIRLY apparent to someone who is pretty experienced with that particular model, but even minor gear mis alignment can be really hard to note.
And, the only way to REALLY check alignment is by landing it. Most mechanics try to do a good job aligning gear during a complete rebuild, but this is NOT something that mechanics check during a pre buy or even an annual.
And, a mis aligns gear can really bedevil you.....they can be really hard to figure out unless they're really ugly.
As to instructors being on the controls, I agree with contact....the student MUST NOT feel the instructor on the controls unless it is to save a situation that the student simply doesn't recognize. And, that's the difference between a GOOD instructor and a good pilot: the good Instructor must GUARD the controls at ALL times, but not actually TOUCH the controls unless the student:
1). Is clearly making a mistake.
2). Clearly does not SEE the error
3). isn't doing anything (or the right thing) to remedy the error.
4). And the instructor intervenes JUST in time to save the day, and perhaps the airplane.
If an instructor intervenes too soon, the student will become frustrated and won't learn anything.
But, next time some experienced pilot (who's not an instructor) tells you he or she can check you out in a new to you airplane, remember this....it MAY work out, but review 1 through 4 above. That's not to suggest that ANY instructor is better equipped to handle these things, but this is the key skill set that an EXPERIENCED instructor develops over time, and that non-instructor pilots generally lack.
This is why I have never liked instructing in taildraggers like the 185, 195, etc. things happen pretty quick, and the instructor doesn't have a lot of time to recognize the problem, give the "student" time to recognize it and react, and then save the day if the student doesn't react appropriately. Training an experienced taildragger pilot in one of these isn't too bad, but initial taildragger training in one isn't my idea of fun....and yes, I've done it.
One note of caution on contactflying's comment about the student's using full control inputs: I've had more than one student SLAM the controls to their limits on landing, one of which sent us through the lights and off the runway.....the guy was six foot seven and had a LOT of leverage on that pedal that I could not overcome. We had a long talk about "dancing lightly" on the pedals after that, which worked fine. Long story there, but that student is now flying large taildraggers regularly and doing well.
So, my point is that, yes, full control inputs can be needed, but locking up on the controls is NOT good, especially if the student is stronger than the instructor. I nearly broke a student's nose one day when he locked up on me during a full flap stall in a 150....

He held the yoke ALL the way back, and full right aileron. I popped him in the nose with the back of my left hand, and magically, both his hands went right to his snout. I opted not to fly with him after that.
MTV