Unpredictable "Multiple controls with the same ID 'ctl35' were found." when dynamically adding controls
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I understand that this question has popped up countless times but none of the posts I can find have been able to help me with this issue.

Every once in a while, I receive the error in the title: `Multiple controls with the same ID 'ctl35' were found. FindControl requires that controls have unique IDs.` This certainly does not happen all the time, and it seems that simply refreshing a few times will cause the error to go away.

My page essentially has an `UpdatePanel` and in it, the user is meant to enter data corresponding to parts of an order. Since there can be multiple parts, there is a button that creates a new `Panel`, creates input fields and adds it to the `Panel`, and then adds it to the `UpdatePanel`. A user can click this button multiple times to add multiple entry `Panel`s. In order to accomplish this, I maintain a `List<Panel>` which I add the new `Panel` to every time the user wants a new entry field. When the page receives a postback (from the add button), I then iterate over the List and re-add each `Panel` to the `UpdatePanel`, which manages to persist the existing entry Panels.

Since these fields are dynamically updated, I made sure to set the IDs of the fields that I'll be retrieving data from to those that will be unique and distinguishable from each other. However, as mentioned, I will occasionally run into issues about finding multiple controls with an ASP.NET auto-generated ID (`ctl35` in this case). Looking at the rendered HTML in Chrome once I receive this error, there simply are no controls that have `ctl35` as their ID. Furthermore, none of the rendered HTML even contains any elements whose IDs end in the auto-generated "ctlXX". Because of this, I can't understand where a possible named conflict issue can come in. The fact that this error doesn't occur most of the time also is a major source of confusion here.

Any ideas on what this issue may be, or what I might need to do in order to troubleshoot this?