After upgrading to the new Square_webPay 1.2.1 I found a bug in (I think) OPC. While checking out and paying with Square_webPay, OR check/money order but NOT Paypal Restfull, If I change what's in the special Instructions or Comments field (or whatever 'TABLE_HEADING_COMMENTS' was defined to originally) I get the error "Your orders details have changed. Please review the current values and re-submit. I had had trouble with this previously with paypal, but not connected it to the comments field.
ZenCart - 1.5.8a
Bootstrap - 3.6.0
OPC - 2.5.1
Square_webPay (paid version) - 1.2.1
PayPal Checkout (RESTful) - 1.0.2

Hoping to upgrade OCP to 2.5.2 today, will post if it still happens in that version.