I would highly recommend watching this video carefully regarding a sin that sahaba (RA) fell into until our beloved Prophet Muhammad (SAWS) called them out for it and warned them and gave them advice.
So what exactly is the major sin that could be done in Wudu? Watch the video that mentions Sahih Hadith.
One of the Hadith mentioned in this video is as follows:
Narrated `Abdullah bin `Amr:
The Prophet (ﷺ) remained behind us on a journey. He joined us while we were performing ablution for the `Asr prayer which was overdue and we were just passing wet hands over our feet (not washing them thoroughly) so he addressed us in a loud voice saying twice, “Save your heels from the fire.” – Sahih al-Bukhari 163
Even today, people might have been committing this sin unknowingly or due to a lack of knowledge during Wudu. Not just feet, it is extremely important for every Muslim to clean every part thoroughly during Wudu. I have seen many people rushing out for Salah without performing Wudu properly and that MUST be avoided at all cost.
Also, know the Dua to read after wudu.
