There are a lot of crawler 'tricks', but the upgrade to cash in one is what I think you mean. Simply, crawlers can be upgraded with armor/abilities, which increases their mineral value. Since it's generally always cheaper to upgrade a unit than it is to rush-buy something, especially a SP, one can simply upgrade a regular crawler to armored and then cash it in for the new improved mineral value. This is certainly an exploit, though it's not really a problem of game mechanics. It's perfectly legitimate to upgrade units, and it's legitimate to cash in crawlers (which is what the AI uses them for, mostly). However, I don't think the game designers intended this work around for cheapening the price of rush-building.
-Smack
-Smack
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