A number of vehicles had to be revised to work with current construction rules, mostly due to standardizing how you round tonnages, but also because of other rules changes.
This means that stats from older products might not necessarily be entirely valid. This is pretty much limited to some of the vehicles in the original TRO 3025 and 3026 (including later version of TRO 3026, when the discrepancy was noted and specifically ignored), the spacecraft stats in the original dropships and jumpships sourcebook and the unrevised TRO 3057, and certain vehicles you might find in vintage sourcebooks or recordsheet books (first edition mechwarrior, the record sheet variants books).
It's worth noting, however, that almost every one of these "illegal" units has been revised in current publications and are currently legal. Even the ones that haven't been revised yet is more a case of the powers that be not getting around to them yet rather than any effort to pretend they don't exist.
So for the most part, if it appears in a catalyst published book, you can count on it being legal. The only exception I can think of at the moment is the mobile longtom, but you can download a corrected version of its record sheet
on the downloads page of the battletech website.
