Question: We are designing a flow where the user is expected to enter some additional mandatory data after entering some other data. For example, the user enters amount in a field and does a TAB-OUT. We want the system to automatically pop-up a modal window to tell the system certain mandatory information about the source of funds. It is agreed by everyone that modal window is the best option.

Now, the question: Should the system automatically pop-up a modal window after the user does a TAB-OUT or provide an icon and ask the user to click the icon? One view is, automatic pop-up is better because the step is sequential, mandatory (and hence expected in a sense). It also saves a click. Contrary view is any automatic pop-up is bad since it is "unexpected" for the user since no explicit action was initiated by the user. 

The first camp argues that "unexpected" pop-ups are bad when it is used for messages that are meant to solicit unrelated inputs (like advertisements, donations etc) and not when used in an enterprise application where it is an essential step in the process.

A quick background on the nature of system: The system is meant for internal users of an enterprise and therefore the users will be repeating the task multiple times on a day to day basis. The usage is primarily on desktop.

What is the right way to go in this context?

Answer: 
Yes, this **PROBABLY is an anti pattern**. Please note the PROBABLY word, because anti-patterns are usually measured on client's side. But given the information you provided and with some experience on my back, I'd bet money this is like the definition of an anti pattern: something that looks like a great idea at first, but use demonstrates it's counter-productive or provides a bad experience to users (like in your case)

Let's look to a known example: **credit card numbers.** There are lot of studies about the proper UX of a string which **only has number and a known pattern** . Even on this site, you'll find lots of question about this, including whether to use a single input, use masked inputs, use 4 different inputs, auto tab and so on.

Now, your example: it has **2 extremely different** data types, with no clue on affordance and everything in the same field. Furthermore: if you don't comply with the mysterious UX, your input doesn't validate. And as you say, even on *validation error messages*, it's unclear what to do. 

Now, compare both cases: one is way simpler, has a lot of studies about it and still represents a problem sometimes, including some anti patterns here and there. The other is complicated, with obfuscated data, unclear for the user and no clues on affordance, with botched validation messages. So.... what would you say about this? *(I know what would you say because you said it in your question, and you're 100% right, it's a rhetorical question)*

In short: as I said, **I don't have the client's side data to confirm this is an anti pattern** but... *If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family Anatidae on our hands.*

EDIT:
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On a second view, now I can see they can share this payment on Facebook... guess that's the explanation for the convoluted UX, but well... posting your financial movements on Facebook is weird to say the least. As a matter of fact, this is the weirdest part of this UX for me
I don't think it is the best pattern here but it is not an anti pattern. If a field did not have mixed data there would be not need for parsers. 

* query statement for a database (SQL)
* programming language
* Number of document management applications use a search syntax with
multiple field types. It is part of them CMIS specification.
* Many of the litigation support applications use that syntax
* Westlaw syntax is common
* [Jason](http://www.json.org/)
* [ODATA](http://www.odata.org/documentation/odata-version-2-0/uri-conventions/)
* [LUCENE](https://lucene.apache.org/core/2_9_4/queryparsersyntax.html) (the leading search engine)