Steps for reproducable bug:

create a rectangle
convert to spline
create a gridhelper
activate the grid

switch to ortographic view if you weren’t allready
go into spline subobject
refine somewhere along the spline ...

it STILL doesn’t refine it correctly (ak precicely where I click the segment. it just places it randomly along it)
it does work in perspective, but not in ortographic view

This bug has been around since at least R7

I tried Maneswars workaround, which still works (using an edit spline modifier) but I hate workarounds as much as the next guy.

After so many versions, it should be fixed.
perhaps no-one ever thought to submit this bug ?

I tried submitting it as a bug, but the bugsubmissionform was broken at the time :(

3ds Max 2009 Design - XpPro 32bit SP2
Dual Intel Xeon 2.3GHz - 4Gb Ram - nVidia GeForce 7600gt

Defect Report Submission Form seems to be working now.

Max 4.2 through 2013.
XP-64 (SP2)
NVidia 9800GTX-512 (Driver 266.58).
Core 2 Quad Q6600 2.4GHz, 8Gb Ram, DX9.0c.

I can confirm that it goes wrong with a grid helper active. IIRC the original bug had nothing to do with grid helpers - it just didn’t work correctly at all unless you added the Edit Spline modifier.

Max 4.2 through 2013.
XP-64 (SP2)
NVidia 9800GTX-512 (Driver 266.58).
Core 2 Quad Q6600 2.4GHz, 8Gb Ram, DX9.0c.

I think the problem was caused by the snap tools. Turn it off. It will work.