Have you sightseen lately?
In recent lexicographic work, I had occasion to think about the meaning of words related to tourism, including sightseeing. Now, to a fair number of people, I think, sightseeing is a noun (or verb) that appears only in the -ing form. That is, I want to sightsee or something like that would be bad. But I think for many (perhaps even a larger number of) people, sightsee is a perfectly fine O-V compound, akin to housekeep, globe-trot, hero-worship, and stage-manage, typewrite, hogtie, proofread, brainwash, vacuum clean, and top-dress. Of course, some of these are likely no longer thought of as having this particular structure (like perhaps proofread) and some are mostly obsolete or rather domain-specific (typewrite and top-dress).
This particular pattern of compounding is actually rather rare in English. I found a paper by Hans Marchand, Compound and Pseudo-Compound Verbs in Present-Day English (American Speech, 1957), which is a great discussion of the morphology and semantics of various sorts of compounds in English. Most of the words above are from that paper, since, as you might have realized, this is not exactly a very productive process for creating verbs in English. Several of the above words have likely been backformed from nominal compounds: sightseeing, stage manager, and so forth. These sorts of compounds, schematically O-V-er/ing, are basically freely-created in English (blog-writer, biscuit-baking, though ??blog-write and ??biscuit-bake).
In the case of the -ing compounds, like (for some people) sight-seeing, these can of course be used as nouns, but they can also go where verbs are supposed to go, as in I don’t like him sightseeing all the time when we’re here for business (at least, this according to one person I asked, who liked this sentence but is against I like to sightsee). In some cases, I would guess, this sort of syntactic context helps convince people that the other inflectional forms are available. For me, I’m pretty comfortable with the bare form: to sightsee. The 3sg-present form, sightsees should in principle be fine, but actually it seems a little odd to use it. A cursory search of Google reveals that the -s form is far less common (and most of the first pages of results are not actually uses of the word at all). Then comes the plain past and participial forms: sightsaw and sightseen (or is it sightseed?). These, I think we can all agree, are ridiculous.
I was pleased to find that languagehat had alread covered the topic, with all present in agreement that go sightseeing is the expression you want if you feel like adding tense morphology. Nonetheless the horrible horrible forms have made their way into the dictionaries. Maybe some sort of automated wordform-generator accidentally word-generated them. Interestingly, there are some websites written in Chinese and Korean (among others, surely, but those are the ones I found) that have discussions of the bad wordforms, and also what seem to be essays from English classes that make use of them. Just goes to show what you get for trusting a dictionary… uh, not to poke undue fun at dictionaries, which I assume are trustworthy, at least in the realm of spelling and wordforms.