Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.047.

The following invalidates the wiping:

1. The end of the permissible time period for wiping.

2. Post-sex impurity.

3. Removal of the socks. If (i) or (iii) occurs while the person was in a state of purity, he need only wash his feet.