Record data type, max reqd size, etc.
Two things come to mind
1- String of characters you are entering is greater than 255 (MS predefined max length) This can be altered in the properties. Or greater than a length you may have changed it too. I got caught out here before as when setting properties accidentally removed a 5 and only gave my text box max length of 25.
2- Your users are attempting to use a stretched text box for data entry where a memo field would be more beneficial