A little over a month ago, I posted a link to Kroonen's article on five new proposed pre-Proto-Germanicisms here. Or to put it into plainer English, I posted a link to an article by Kroonen that argues that he has discovered five new words in Germanic languages that do not come from Proto-Germanic's ancestor, Proto-Indo-European, but rather come from an unrelated European language that died out prior to writing. His overall argument is that the words were loanwords because they come from a farming culture and the invasive Indo-Europeans did not have as developed an agriculture system. With work done I finally got around to reading it.
To make the case that they are not native lexical items to Germanic languages, Kroonen borrows from Schrijver's a-~0-prefixation argument. You can see examples abound in the Pre-Proto-Germanic section of my dictionary. If a word began with a-, it eliminated the sequential vowel and reduced the consonants to a cluster: e.g., Old High Germanic amsala "blackbird" vs. Latin mesal. In the Old High Germanic cognate, the -e- has disappeared and -mes- became a cluster -ms-. This has not an Indo-European feature, yet the feature is so strongly attested in Western European languages that it surely came from a non-Indo-European source.
It has occurred to me that Linear A, while undeciphered presently, bears a strange prefixation pattern with i-. Some that come to mind
da-ma-te (probably Demeter) ~ i-da-ma-te
a-ta-i-301-... ~ ja-ta-i-301-... (i-a-ta-i-301-...?)
a-sa-sa-ra ~ ja-sa-sa-ra (i-a-sa-sa-ra?)
a-di-ki-te (probably Dikte) ~ ja-di-ki-te (i-a-di-ki-te?)
zu-ri-ni-ma ~ i-zu-ri-ni-ta
pa-sa-ja ~ i-pa-sa-ja
And possibly a- as well:
tu-ri-sa ~ a-tu-ri-si-ti
Vocalic prefixation also crops up in Pre-Greek (via Beekes) as a-, (e.g., kirris ~ akiris).