Whenever a commit happens in Solr, a new "searcher" (with new caches) is opened, "warmed" up according to your SolrConfigXml settings, and then put in place. The previous searcher is not closed until the "warming" search is ready. If multiple commits happen in rapid succession -- before the warming searcher from first commit has enough time to warm up, then there can be multiple searchers all competeing for resources at the same time, even htough one of them will be thrown away as soon as the next one is ready.
maxWarmingSearchers is a setting in SolrConfigXml that helps you put a safety valve on the number of overlapping warming searchers that can exist at one time. If you see this error it means Solr prevented a commit from resulting an a new searcher being opened because there were already X warming searchers open.
If you encounter this error a lot, you can (in theory) increase the number in your maxWarmingSearchers, but that is risky to do unless you are confident you have the system resources (RAM, CPU, etc...) to do it safely. A more correct way to deal with the situation is to reduce how frequently you send commits.
If you only encounter this error infrequently because of fluke situations, you'll probably be ok just ignoring it.
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http://wiki.apache.org/solr/FAQ#What_does_.22exceeded_limit_of_maxWarmingSearchers.3DX.22_mean.3F