A brief search did not return this issue - In Collector if a user incorrectly inputs text into a numeric attribute field there is no warning, and the user can proceed to complete any additional data entry and submit the feature or updates. The submission completes, however the incorrect input is not saved the fields are returned to null/blank - and the user may not realize this is happening. I have observed this at least for ArcGIS Server feature services.
It would be good to have a warning or check in place to prevent the user from unknowingly inputting data that will not save. For example in the basic web map viewer you get the warning shown:
Please let me know if I'm overlooking something here.