Subject verb disagreements like this are becoming increasingly common in English. It's as though the writer or speaker looks at the last noun they've used and makes the verb agree with that, instead of searching for the subject of the sentence. In this case, it is the "illuminated red beam on the doors" that should change to green, not the doors themselves, so the sentence should read:
Please do not enter lift cart until the illuminated red beam on the doors changes to green.
Am I being unnecessarily pedantic? I don't think so. The UK Guardian newspaper recently published an article about the Idler Academy's Bad Grammar Awards. Stray apostrophes feature there, as well as confusing subjects/objects and inaccurate spelling.
Do these things really matter, though? Surely communication is the main thing? After all, language is all about communication. Yes and no. Spelling mistakes may not matter in the long run, but if communication is impeded by bad grammar or punctuation then there may be serious problems. Think about these sentences from the Cybertext Newsletter blog:
Most of the time travellers worry about their luggage.
Most of the time, travellers worry about their luggage.
Are we talking about ordinary travellers or time travellers?
One of my other favourites is this Australian sign:
Without the punctuation, this could mean that drivers should go more slowly because children could be injured if a car hit them. However it can also mean that children should walk more slowly and not bounce along the road!
Finally, let's look at another sign:
The top line starts, "You would of noticed . . ." When we speak in English we usually say "would've". The writer here obviously didn't realise this sound meant "would have" and not "would of".
Little mistakes like this don't matter in terms of communication, but they make the reader think that the writer is either careless or uneducated, and that is not helpful if someone is trying to sell a product, impress a future employer or write a good academic essay.
What common mistakes have you noticed around you, and have your own writing mistakes ever had a comic or serious effect in your life?