When reading fast, we tend to read first/last letters and the 'pattern' of the whole word and not really read the spelling. In this case, Reticulum is not very common or a familiar word and most folks would first read (or rather 'see') the word 'rectum'!

https://pubmed.ncbi.nlm.nih.gov/23089042/ https://www.dictionary.com/e/typoglycemia/

You described the phenomenon much better than I would have.

At first glance the word looks a lot like “rectum”.

I thought it meant "small rectum" as a diminutive[1] :D

[1] https://en.wikipedia.org/wiki/Diminutive