Take a look at the sentence again:
The problem occurs between shirt and all. Rachel painstakingly ironed her linen shirt is a main clause, so it can stand alone as a complete sentence. All the while, she was thinking … begins the next complete sentence. Because these two parts have only blank space between them, they make a fused sentence, a major error.
To fix the problem, you could put a period after shirt and then capitalize the A for all.