Cannot open entity customization form

We were experiencing an issue whereby we could not modify the contact entity web form. Every time we'd try and open up the form editor, the following error message would be issued:

A little bit more information was revealed after turning on DevErrors:

Not sure how this issue came about but I imagine it's likely only to occur in an upgraded environment. Away, after a bit of digging I came across the following article which seemed to describe the same problem (albeit for the lead form):

http://blogs.msdn.com/b/atif/archive/2012/01/22/unable-to-open-lead-entity-form-in-crm-2011.aspx

So I decided to export my contact entity and perform a search on all cases where "rowspan" has a value greater than 1 (e.g. rowspan="4"). I came across 6 cases and in each case I performed a count of the row elements and they matched up with the rowspan count. Until the very last one that is...

In short, the screenshot below is what I found.

The rowspan count equalled to 4 yet there was only one row element (the one containing the rowspan element). So I added in 3 additional row elements as shown in the highlighted text, imported the customization and published.

And just as advertised this resolved the issue.