BIDMAS/BODMAS, and the alternatives the Americans use, are not definitions, rules or anything official, as has been said above. The reason this doesn't work:
18 - 1 + 1
= 18 - 2
is because the rule is a guideline not written in stone. Has this been encountered before? Yes, far too many times since a lot of people get caught up in these being rules not guidelines. The TES maths forum has several long running posts on this issue where otherwise sensible people become very upset about such a simple issue.
You can usually spot when a student has used the calculator on their mobile as it doesn't follow the idea embedded in BODMAS. Of course with everyone having apps on their ipads this happens less frequently now.