Anyone seen this? Field deleting information

I have this computedWhenComposed field called ‘InputHistory’. When a user clicks on ‘Save & Close’ it saves the information from the input field to the input history field with the already stored input history. I have done this programming many times - without any problems.

BUT I have this one database - that will save the information to the input history field - and if the input history field is too large - instead of popping up an error message - it cuts the bottom chunk of the input history field out and saves the input field to it.

Any ideas? Let me know if I have explained this enough for you - thanks.