Yeah, silent failures are waaaaay too common with Velocity scripts in Marketo, unfortunately. I was on the phone with Marketo support yesterday and tried to convince them it was a problem, but of course ... the real-time example I tried to set up didn't actually fail. It's the kind of thing you experience in the moment and, since you don't always know why it happend, it can be hard to replicate.
Marketo seems to do some very basic checking/validation of scripts at two points:
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When an email (or other asset) is published that references the script
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When you send a sample test message
Marketo doesn't seem to check for the same kinds of issues in both 1 and 2, so if you edit a script token after the email where it's used is already published, you can end up with silent failures on your test emails (and presumably on your active campaigns).
Seems like the product has a ways to go to make Velocity scripting a friendlier and more reliable/predictable user experience in Marketo. That said ... I'm still glad it's there. Just wish it were more robust and fully documented.
-patrick