As with many things, "it depends". It depends on what you mean by "incorrect" and it depends on the specific verb.
To generalise hugely, there is a phenomenon whereby children acquiring the language overgeneralise regular patterns while in the fairly stages of acquisition. So a 4 year old child may well say "He catched it". But a normally developed adult native speaker would probably never in their wildest dreams of a slip of the tongue use the form "catched" (unless they were deliberately mimicking child language, or for some other humoristic effect). The same generally goes for common irregular verbs. So in these cases, by pretty much any useful definition of the word, you would say that "catched" and "buyed" are "incorrect".
However, this doesn't hold for all irregular verbs. There are a few verbs 'on the fringe' where for a variety of reasons even in the adult language, there will be variation. This includes regional or ideolectal variation: e.g. "treat" can be irregular in Yorkshire English, whereas it is regular in standard English; "sneak" is generally regular in British English, but typically irregular in US English. You can even get cases where a verb is considered irregular, but there is uncertainty about the actual irregular form (e.g. "He span" vs "He spun"; "He rode it" vs "He rid it", though the latter would generally be considered non-standard). In these cases, whether e.g. "he treat [trEt] them well", "he snuck in", "He rid it fast" are "incorrect" or "correct" depends on what you decide your measure of "correctness" is.
In the case of some relatively rare verbs, it may be that many adult native speakers are simply never explicitly exposed to the 'standard' irregular form, so that they will tend to regularise. The most common past tense of "broadcast" is irregular, but there are a number of tangible instances with the regular form (e.g. "They never broadcast[ed] it"). Indeed, you're unlikely to get much of a consensus about the precise cases where "cast" is regular vs irregular ("They cast[ed] him as the hero"). These are all cases where the variants are probably all used sufficiently in adult, educated usage that you'd be generally hard pushed to say that one is "incorrect" by most definitions.